Overview.

Unix was originally meant to be a convenient platform for programmers developing software to be run on it and on other systems, rather than for non-programmers. The system grew larger as the operating system started spreading in academic circles, and as users added their own tools to the system and shared them with colleagues.  At first, Unix was not designed to be portable or for multi-tasking. Later, Unix gradually gained portability, multi-tasking and multi-user capabilities in a time-sharing configuration. Unix systems are characterized by various concepts: the use of plain text for storing data; a hierarchical file system; treating devices and certain types of inter-process communication (IPC) as files; and the use of a large number of software tools, small programs that can be strung together through a command-line interpreter using pipes, as opposed to using a single monolithic program that includes all of the same functionality. These concepts are collectively known as the "Unix philosophy". Brian Kernighan and Rob Pike summarize this in "The Unix Programming Environment" as "the idea that the power of a system comes more from the relationships among programs than from the programs themselves".  By the early 1980s, users began seeing Unix as a potential universal operating system, suitable for computers of all sizes. The Unix environment and the client–server program model were essential elements in the development of the Internet and the reshaping of computing as centered in networks rather than in individual computers.  Both Unix and the C programming language were developed by AT&T and distributed to government and academic institutions, which led to both being ported to a wider variety of machine families than any other operating system.  The Unix operating system consists of many libraries and utilities along with the master control program, the kernel. The kernel provides services to start and stop programs, handles the file system and other common "low-level" tasks that most programs share, and schedules access to avoid conflicts when programs try to access the same resource or device simultaneously. To mediate such access, the kernel has special rights, reflected in the distinction of "kernel space" from user space, the latter being a priority realm where most application programs operate.

History.

The origins of Unix date back to the mid-1960s when the Massachusetts Institute of Technology, Bell Labs, and General Electric were developing Multics, a time-sharing operating system for the GE-645 mainframe computer. Multics featured several innovations, but also presented severe problems. Frustrated by the size and complexity of Multics, but not by its goals, individual researchers at Bell Labs started withdrawing from the project. The last to leave were Ken Thompson, Dennis Ritchie, Douglas McIlroy, and Joe Ossanna, who decided to reimplement their experiences in a new project of smaller scale. This new operating system was initially without organizational backing, and also without a name.  The new operating system was a single-tasking system. In 1970, the group coined the name "Unics" for "Uniplexed Information and Computing Service" (pronounced "eunuchs"), as a pun on "Multics", which stood for "Multiplexed Information and Computer Services". Brian Kernighan takes credit for the idea, but adds that "no one can remember" the origin of the final spelling "Unix". Dennis Ritchie, Doug McIlroy, and Peter G. Neumann also credit Kernighan.  The operating system was originally written in assembly language, but in 1973, Version 4 Unix was rewritten in C. Version 4 Unix, however, still had many PDP-11 dependent codes, and was not suitable for porting. The first port to another platform was made five years later (1978) for Interdata 8/32.  Bell Labs produced several versions of Unix that are collectively referred to as "Research Unix". In 1975, the first source license for "UNIX" was sold to Donald B. Gillies at the University of Illinois at Urbana–Champaign Department of Computer Science. UIUC graduate student Greg Chesson, who had worked on the UNIX kernel at Bell Labs, was instrumental in negotiating the terms of the license.  During the late 1970s and early 1980s, the influence of Unix in academic circles led to large-scale adoption of Unix (BSD and System V) by commercial startups, including Sequent, HP-UX, SunOS/Solaris, AIX, and Xenix. In the late 1980s, AT&T Unix System Laboratories and Sun Microsystems developed System V Release 4 (SVR4), which was subsequently adopted by many commercial Unix vendors.  In the 1990s, Unix and Unix-like systems grew in popularity and became the operating system of choice for over 90% of the world's top 500 fastest supercomputers, as BSD and Linux distributions were developed through collaboration by a worldwide network of programmers. In 2000, Apple released Darwin, also a Unix system, which became the core of the Mac OS X operating system, which was later renamed macOS.  Unix operating systems are widely used in modern servers, workstations, and mobile devices.

Standards.

In the late 1980s, an open operating system standardization effort now known as POSIX provided a common baseline for all operating systems; IEEE based POSIX around the common structure of the major competing variants of the Unix system, publishing the first POSIX standard in 1988. In the early 1990s, a separate but very similar effort was started by an industry consortium, the Common Open Software Environment (COSE) initiative, which eventually became the Single UNIX Specification (SUS) administered by The Open Group. Starting in 1998, the Open Group and IEEE started the Austin Group, to provide a common definition of POSIX and the Single UNIX Specification, which, by 2008, had become the Open Group Base Specification.  In 1999, in an effort towards compatibility, several Unix system vendors agreed on SVR4's Executable and Linkable Format (ELF) as the standard for binary and object code files. The common format allows substantial binary compatibility among different Unix systems operating on the same CPU architecture.  The Filesystem Hierarchy Standard was created to provide a reference directory layout for Unix-like operating systems; it has mainly been used in Linux.

Components.

The Unix system is composed of several components that were originally packaged together. By including the development environment, libraries, documents and the portable, modifiable source code for all of these components, in addition to the kernel of an operating system, Unix was a self-contained software system. This was one of the key reasons it emerged as an important teaching and learning tool and has had such a broad influence.  The inclusion of these components did not make the system large the original V7 UNIX distribution, consisting of copies of all of the compiled binaries plus all of the source code and documentation occupied less than 10 MB and arrived on a single nine-track magnetic tape. The printed documentation, typeset from the online sources, was contained in two volumes.  The names and filesystem locations of the Unix components have changed substantially across the history of the system. Nonetheless, the V7 implementation is considered by many to have the canonical early structure:

Impact.

The Unix system had a significant impact on other operating systems. It achieved its reputation by its interactivity, by providing the software at a nominal fee for educational use, by running on inexpensive hardware, and by being easy to adapt and move to different machines. Unix was originally written in assembly language, but was soon rewritten in C, a high-level programming language. Although this followed the lead of Multics and Burroughs, it was Unix that popularized the idea.  Unix had a drastically simplified file model compared to many contemporary operating systems: treating all kinds of files as simple byte arrays. The file system hierarchy contained machine services and devices (such as printers, terminals, or disk drives), providing a uniform interface, but at the expense of occasionally requiring additional mechanisms such as ioctl and mode flags to access features of the hardware that did not fit the simple "stream of bytes" model. The Plan 9 operating system pushed this model even further and eliminated the need for additional mechanisms.  Unix also popularized the hierarchical file system with arbitrarily nested subdirectories, originally introduced by Multics. Other common operating systems of the era had ways to divide a storage device into multiple directories or sections, but they had a fixed number of levels, often only one level. Several major proprietary operating systems eventually added recursive subdirectory capabilities also patterned after Multics. DEC's RSX-11M's "group, user" hierarchy evolved into VMS directories, CP/M's volumes evolved into MS-DOS 2.0+ subdirectories, and HP's MPE group.account hierarchy and IBM's SSP and OS/400 library systems were folded into broader POSIX file systems.  Making the command interpreter an ordinary user-level program, with additional commands provided as separate programs, was another Multics innovation popularized by Unix. The Unix shell used the same language for interactive commands as for scripting (shell scripts – there was no separate job control language like IBM's JCL). Since the shell and OS commands were "just another program", the user could choose (or even write) their own shell. New commands could be added without changing the shell itself. Unix's innovative command-line syntax for creating modular chains of producer-consumer processes (pipelines) made a powerful programming paradigm (coroutines) widely available. Many later command-line interpreters have been inspired by the Unix shell.  A fundamental simplifying assumption of Unix was its focus on newline-delimited text for nearly all file formats. There were no "binary" editors in the original version of Unix – the entire system was configured using textual shell command scripts. The common denominator in the I/O system was the byte – unlike "record-based" file systems. The focus on text for representing nearly everything made Unix pipes especially useful and encouraged the development of simple, general tools that could be easily combined to perform more complicated "ad hoc" tasks. The focus on text and bytes made the system far more scalable and portable than other systems. Over time, text-based applications have also proven popular in application areas, such as printing languages (PostScript, ODF), and at the application layer of the Internet protocols, e.g., FTP, SMTP, HTTP, SOAP, and SIP.  Unix popularized a syntax for regular expressions that found widespread use. The Unix programming interface became the basis for a widely implemented operating system interface standard (POSIX, see above). The C programming language soon spread beyond Unix, and is now ubiquitous in systems and applications programming.  Early Unix developers were important in bringing the concepts of modularity and reusability into software engineering practice, spawning a "software tools" movement. Over time, the leading developers of Unix (and programs that ran on it) established a set of cultural norms for developing software, norms which became as important and influential as the technology of Unix itself; this has been termed the Unix philosophy.  The TCP/IP networking protocols were quickly implemented on the Unix versions widely used on relatively inexpensive computers, which contributed to the Internet explosion of worldwide real-time connectivity, and which formed the basis for implementations on many other platforms.  The Unix policy of extensive on-line documentation and (for many years) ready access to all system source code raised programmer expectations, and contributed to the launch of the free software movement in 1983.

Free Unix and Unix-like variants.

In 1983, Richard Stallman announced the GNU (short for "GNU's Not Unix") project, an ambitious effort to create a free software Unix-like system; "free" in the sense that everyone who received a copy would be free to use, study, modify, and redistribute it. The GNU project's own kernel development project, GNU Hurd, had not yet produced a working kernel, but in 1991 Linus Torvalds released the kernel Linux as free software under the GNU General Public License. In addition to their use in the GNU operating system, many GNU packages – such as the GNU Compiler Collection (and the rest of the GNU toolchain), the GNU C library and the GNU core utilities – have gone on to play central roles in other free Unix systems as well.  Linux distributions, consisting of the Linux kernel and large collections of compatible software have become popular both with individual users and in business. Popular distributions include Red Hat Enterprise Linux, Fedora, SUSE Linux Enterprise, openSUSE, Debian GNU/Linux, Ubuntu, Linux Mint, Mandriva Linux, Slackware Linux, Arch Linux and Gentoo.  A free derivative of BSD Unix, 386BSD, was released in 1992 and led to the NetBSD and FreeBSD projects. With the 1994 settlement of a lawsuit brought against the University of California and Berkeley Software Design Inc. ("USL v. BSDi") by Unix System Laboratories, it was clarified that Berkeley had the right to distribute BSD Unix for free if it so desired. Since then, BSD Unix has been developed in several different product branches, including OpenBSD and DragonFly BSD.  Linux and BSD are increasingly filling the market needs traditionally served by proprietary Unix operating systems, as well as expanding into new markets such as the consumer desktop and mobile and embedded devices. Because of the modular design of the Unix model, sharing components is relatively common; consequently, most or all Unix and Unix-like systems include at least some BSD code, and some systems also include GNU utilities in their distributions.  In a 1999 interview, Dennis Ritchie voiced his opinion that Linux and BSD operating systems are a continuation of the basis of the Unix design, and are derivatives of Unix:  In the same interview, he states that he views both Unix and Linux as "the continuation of ideas that were started by Ken and me and many others, many years ago".  OpenSolaris was the open-source counterpart to Solaris developed by Sun Microsystems, which included a CDDL-licensed kernel and a primarily GNU userland. However, Oracle discontinued the project upon their acquisition of Sun, which prompted a group of former Sun employees and members of the OpenSolaris community to fork OpenSolaris into the illumos kernel. As of 2014, illumos remains the only active open-source System V derivative.

ARPANET.

In May 1975, RFC 681 described the development of "Network Unix" by the Center for Advanced Computation at the University of Illinois at Urbana–Champaign. The system was said to "present several interesting capabilities as an ARPANET mini-host". At the time Unix required a license from Bell Laboratories that at $20,000(US) was very expensive for non-university users, while an educational license cost just $150. It was noted that Bell was "open to suggestions" for an ARPANET-wide license.  Specific features found beneficial were the local processing facilities, compilers, editors, a document preparation system, efficient file system and access control, mountable and unmountable volumes, unified treatment of peripherals as special files, integration of the network control program (NCP) within the Unix file system, treatment of network connections as special files that can be accessed through standard Unix I/O calls, closing of all files on program exit, and the decision to be "desirable to minimize the amount of code added to the basic Unix kernel".

Branding.

In October 1993, Novell, the company that owned the rights to the Unix System V source at the time, transferred the trademarks of Unix to the X/Open Company (now The Open Group), and in 1995 sold the related business operations to Santa Cruz Operation (SCO). Whether Novell also sold the copyrights to the actual software was the subject of a federal lawsuit in 2006, "SCO v. Novell", which Novell won. The case was appealed, but on August 30, 2011, the United States Court of Appeals for the Tenth Circuit affirmed the trial decisions, closing the case. Unix vendor SCO Group Inc. accused Novell of slander of title.  The present owner of the trademark "UNIX" is The Open Group, an industry standards consortium. Only systems fully compliant with and certified to the Single UNIX Specification qualify as "UNIX" (others are called "Unix-like").  By decree of The Open Group, the term "UNIX" refers more to a class of operating systems than to a specific implementation of an operating system; those operating systems which meet The Open Group's Single UNIX Specification should be able to bear the UNIX 98 or UNIX 03 trademarks today, after the operating system's vendor pays a substantial certification fee and annual trademark royalties to The Open Group. Systems that have been licensed to use the UNIX trademark include AIX, EulerOS, HP-UX, Inspur K-UX, IRIX, macOS, Solaris, Tru64 UNIX (formerly "Digital UNIX", or OSF/1), and z/OS. Notably, EulerOS and Inspur K-UX are Linux distributions certified as UNIX 03 compliant.  Sometimes a representation like "Un*x", "*NIX", or "*N?X" is used to indicate all operating systems similar to Unix. This comes from the use of the asterisk ("*") and the question mark characters as wildcard indicators in many utilities. This notation is also used to describe other Unix-like systems that have not met the requirements for UNIX branding from the Open Group.  The Open Group requests that "UNIX" is always used as an adjective followed by a generic term such as "system" to help avoid the creation of a genericized trademark.  "Unix" was the original formatting, but the usage of "UNIX" remains widespread because it was once typeset in small caps ("Unix"). According to Dennis Ritchie, when presenting the original Unix paper to the third Operating Systems Symposium of the American Association for Computing Machinery (ACM), "we had a new typesetter and "troff" had just been invented and we were intoxicated by being able to produce small caps". Many of the operating system's predecessors and contemporaries used all-uppercase lettering, so many people wrote the name in upper case due to force of habit. It is not an acronym.  Trademark names can be registered by different entities in different countries and trademark laws in some countries allow the same trademark name to be controlled by two different entities if each entity uses the trademark in easily distinguishable categories. The result is that Unix has been used as a brand name for various products including bookshelves, ink pens, bottled glue, diapers, hair driers and food containers.  Several plural forms of Unix are used casually to refer to multiple brands of Unix and Unix-like systems. Most common is the conventional "Unixes", but "Unices", treating Unix as a Latin noun of the third declension, is also popular. The pseudo-Anglo-Saxon plural form "Unixen" is not common, although occasionally seen. Sun Microsystems, developer of the Solaris variant, has asserted that the term "Unix" is itself plural, referencing its many implementations.

Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, development starting in the 1970s at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and others.

Unix, officiellement UNIX (parfois écrit « Unix »), est une famille de systèmes d'exploitation multitâche et multi-utilisateur dérivé du Unix d'origine créé par AT&T, le développement de ce dernier ayant commencé dans les années 1970 au centre de recherche de Bell Labs mené par Kenneth Thompson. Il repose sur un interpréteur ou superviseur (le "shell") et de nombreux petits utilitaires, accomplissant chacun une action spécifique, commutables entre eux (mécanisme de « redirection ») et appelés depuis la ligne de commande.


Primary units.

In 1928, Ralph Hartley observed a fundamental storage principle, which was further formalized by Claude Shannon in 1945: the information that can be stored in a system is proportional to the logarithm of "N" possible states of that system, denoted. Changing the base of the logarithm from "b" to a different number "c" has the effect of multiplying the value of the logarithm by a fixed constant, namely. Therefore, the choice of the base "b" determines the unit used to measure information. In particular, if "b" is a positive integer, then the unit is the amount of information that can be stored in a system with "N" possible states.  When "b" is 2, the unit is the shannon, equal to the information content of one "bit" (a portmanteau of binary digit). A system with 8 possible states, for example, can store up to log8 = 3 bits of information. Other units that have been named include:  The trit, ban, and nat are rarely used to measure storage capacity; but the nat, in particular, is often used in information theory, because natural logarithms are mathematically more convenient than logarithms in other bases.

Units derived from bit.

Several conventional names are used for collections or groups of bits.

Byte.

Historically, a byte was the number of bits used to encode a character of text in the computer, which depended on computer hardware architecture; but today it almost always means eight bits – that is, an octet. A byte can represent 256 (2) distinct values, such as non-negative integers from 0 to 255, or signed integers from −128 to 127. The IEEE 1541-2002 standard specifies "B" (upper case) as the symbol for byte (IEC 80000-13 uses "o" for octet in French, but also allows "B" in English, which is what is actually being used). Bytes, or multiples thereof, are almost always used to specify the sizes of computer files and the capacity of storage units. Most modern computers and peripheral devices are designed to manipulate data in whole bytes or groups of bytes, rather than individual bits.

Nibble.

A group of four bits, or half a byte, is sometimes called a nibble or nybble. This unit is most often used in the context of hexadecimal number representations, since a nibble has the same amount of information as one hexadecimal digit.

Crumb.

A pair of two bits or a quarter byte was called a crumb, often used in early 8-bit computing (see Atari 2600, ZX Spectrum). It is now largely defunct.

Word, block, and page.

Computers usually manipulate bits in groups of a fixed size, conventionally called "words". The number of bits in a word is usually defined by the size of the registers in the computer's CPU, or by the number of data bits that are fetched from its main memory in a single operation. In the IA-32 architecture more commonly known as x86-32, a word is 16 bits, but other past and current architectures use words with 4, 8, 9, 12, 13, 16, 18, 20, 21, 22, 24, 25, 26, 29, 30, 31, 32, 33, 35, 36, 38, 39, 40, 42, 44, 48, 50, 52, 54, 56, 60, 64, 72, 80 bits or others.  Some machine instructions and computer number formats use two words (a "double word" or "dword"), or four words (a "quad word" or "quad").  Computer memory caches usually operate on "blocks" of memory that consist of several consecutive words. These units are customarily called "cache blocks", or, in CPU caches, "cache lines".  Virtual memory systems partition the computer's main storage into even larger units, traditionally called "pages".

Systematic multiples.

Terms for large quantities of bits can be formed using the standard range of SI prefixes for powers of 10, e.g., kilo = 10 = 1000 (as in kilobit or kbit), mega = 10 = (as in megabit or Mbit) and giga = 10 = (as in gigabit or Gbit). These prefixes are more often used for multiples of bytes, as in kilobyte (1 kB = 8000 bit), megabyte (1 MB = ), and gigabyte (1 GB = ).  However, for technical reasons, the capacities of computer memories and some storage units are often multiples of some large power of two, such as 2 = bytes. To avoid such unwieldy numbers, people have often repurposed the SI prefixes to mean the nearest power of two, e.g., using the prefix "kilo" for 2 = 1024, "mega" for 2 =, and "giga" for 2 =, and so on. For example, a random access memory chip with a capacity of 2 bytes would be referred to as a 256-megabyte chip. The table below illustrates these differences.  In the past, uppercase "K" has been used instead of lowercase "k" to indicate 1024 instead of 1000. However, this usage was never consistently applied.  On the other hand, for external storage systems (such as optical discs), the SI prefixes were commonly used with their decimal values (powers of 10). There have been many attempts to resolve the confusion by providing alternative notations for power-of-two multiples. In 1998 the International Electrotechnical Commission (IEC) issued a standard for this purpose, namely a series of binary prefixes that use 1024 instead of 1000 as the main radix:  The JEDEC memory standards however define uppercase K, M, and G for the binary powers 2, 2 and 2 to reflect common usage.

Obsolete and unusual units.

Several other units of information storage have been named:  Some of these names are jargon, obsolete, or used only in very restricted contexts.

In computing and telecommunications, a unit of information is the capacity of some standard data storage system or communication channel, used to measure the capacities of other systems and channels. In information theory, units of information are also used to measure the entropy of random variables and information contained in messages.

Les unités de mesure suivantes sont utilisées en informatique pour quantifier la taille de la mémoire d'un dispositif numérique (ordinateur, Baladeur numérique), l'espace utilisable sur un disque dur, une clé USB, la taille d'un fichier, d'un répertoire ou autre.


Classification.

Components can be classified as passive, active, or electromechanic. The strict physics definition treats passive components as ones that cannot supply energy themselves, whereas a battery would be seen as an active component since it truly acts as a source of energy.  However, electronic engineers who perform circuit analysis use a more restrictive definition of passivity. When only concerned with the energy of signals, it is convenient to ignore the so-called DC circuit and pretend that the power supplying components such as transistors or integrated circuits is absent (as if each such component had its own battery built in), though it may in reality be supplied by the DC circuit. Then, the analysis only concerns the AC circuit, an abstraction that ignores DC voltages and currents (and the power associated with them) present in the real-life circuit. This fiction, for instance, lets us view an oscillator as "producing energy" even though in reality the oscillator consumes even more energy from a DC power supply, which we have chosen to ignore. Under that restriction, we define the terms as used in circuit analysis as:  Most passive components with more than two terminals can be described in terms of two-port parameters that satisfy the principle of reciprocity—though there are rare exceptions. In contrast, active components (with more than two terminals) generally lack that property.

Active components.

Semiconductors.

Transistors.

Transistors were considered the invention of the twentieth century that changed electronic circuits forever. A transistor is a semiconductor device used to amplify and switch electronic signals and electrical power.

Diodes.

Conduct electricity easily in one direction, among more specific behaviors.

Display technologies.

Current: Obsolete:

Vacuum tubes (valves).

A vacuum tube is based on current conduction through a vacuum (see Vacuum tube).  Optical detectors or emitters

Discharge devices.

Obsolete:

Power sources.

Sources of electrical power:

Passive components.

Components incapable of controlling current by means of another electrical signal are called "passive" devices. Resistors, capacitors, inductors, and transformers are all considered passive devices.

Resistors.

Pass current in proportion to voltage (Ohm's law) and oppose current.

Capacitors.

Capacitors store and release electrical charge. They are used for filtering power supply lines, tuning resonant circuits, and for blocking DC voltages while passing AC signals, among numerous other uses.

Magnetic (inductive) devices.

Electrical components that use magnetism in the storage and release of electrical charge through current:

Memristor.

Electrical components that pass charge in proportion to magnetism or magnetic flux, and have the ability to retain a previous resistive state, hence the name of Memory plus Resistor.

Networks.

Components that use more than one type of passive component:

Antennas.

Antennas transmit or receive radio waves

Assemblies, modules.

Multiple electronic components assembled in a device that is in itself used as a component

Electromechanical.

Piezoelectric devices, crystals, resonators.

Passive components that use piezoelectric effect:

Terminals and connectors.

Devices to make electrical connection

Cable assemblies.

Electrical cables with connectors or terminals at their ends

Switches.

Components that can pass current ("closed") or break the current ("open"):

Protection devices.

Passive components that protect circuits from excessive currents or voltages:

Standard symbols.

On a circuit diagram, electronic devices are represented by conventional symbols. Reference designators are applied to the symbols to identify the components.

An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are not to be confused with electrical elements, which are conceptual abstractions representing idealized electronic components.

Un composant électronique est un élément destiné à être assemblé avec d'autres afin de réaliser une ou plusieurs fonctions électroniques. Les composants forment de très nombreux types et catégories, ils répondent à divers standards de l'industrie aussi bien pour leurs caractéristiques électriques que pour leurs caractéristiques géométriques.


Principle.

The specimen is illuminated with light of a specific wavelength (or wavelengths) which is absorbed by the fluorophores, causing them to emit light of longer wavelengths (i.e., of a different color than the absorbed light). The illumination light is separated from the much weaker emitted fluorescence through the use of a spectral emission filter. Typical components of a fluorescence microscope are a light source (xenon arc lamp or mercury-vapor lamp are common; more advanced forms are high-power LEDs and lasers), the excitation filter, the dichroic mirror (or dichroic beamsplitter), and the emission filter (see figure below). The filters and the dichroic beamsplitter are chosen to match the spectral excitation and emission characteristics of the fluorophore used to label the specimen. In this manner, the distribution of a single fluorophore (color) is imaged at a time. Multi-color images of several types of fluorophores must be composed by combining several single-color images.  Most fluorescence microscopes in use are epifluorescence microscopes, where excitation of the fluorophore and detection of the fluorescence are done through the same light path (i.e. through the objective). These microscopes are widely used in biology and are the basis for more advanced microscope designs, such as the confocal microscope and the total internal reflection fluorescence microscope (TIRF).

Epifluorescence microscopy.

The majority of fluorescence microscopes, especially those used in the life sciences, are of the epifluorescence design shown in the diagram. Light of the excitation wavelength illuminates the specimen through the objective lens. The fluorescence emitted by the specimen is focused to the detector by the same objective that is used for the excitation which for greater resolution will need objective lens with higher numerical aperture. Since most of the excitation light is transmitted through the specimen, only reflected excitatory light reaches the objective together with the emitted light and the epifluorescence method therefore gives a high signal-to-noise ratio. The dichroic beamsplitter acts as a wavelength specific filter, transmitting fluoresced light through to the eyepiece or detector, but reflecting any remaining excitation light back towards the source.

Light sources.

Fluorescence microscopy requires intense, near-monochromatic, illumination which some widespread light sources, like halogen lamps cannot provide. Four main types of light source are used, including xenon arc lamps or mercury-vapor lamps with an excitation filter, lasers, supercontinuum sources, and high-power LEDs. Lasers are most widely used for more complex fluorescence microscopy techniques like confocal microscopy and total internal reflection fluorescence microscopy while xenon lamps, and mercury lamps, and LEDs with a dichroic excitation filter are commonly used for widefield epifluorescence microscopes. By placing two microlens arrays into the illumination path of a widefield epifluorescence microscope, highly uniform illumination with a coefficient of variation of 1-2% can be achieved.

Sample preparation.

In order for a sample to be suitable for fluorescence microscopy it must be fluorescent. There are several methods of creating a fluorescent sample; the main techniques are labelling with fluorescent stains or, in the case of biological samples, expression of a fluorescent protein. Alternatively the intrinsic fluorescence of a sample (i.e., autofluorescence) can be used. In the life sciences fluorescence microscopy is a powerful tool which allows the specific and sensitive staining of a specimen in order to detect the distribution of proteins or other molecules of interest. As a result, there is a diverse range of techniques for fluorescent staining of biological samples.

Biological fluorescent stains.

Many fluorescent stains have been designed for a range of biological molecules. Some of these are small molecules which are intrinsically fluorescent and bind a biological molecule of interest. Major examples of these are nucleic acid stains such as DAPI and Hoechst (excited by UV wavelength light) and DRAQ5 and DRAQ7 (optimally excited by red light) which all bind the minor groove of DNA, thus labeling the nuclei of cells. Others are drugs, toxins, or peptides which bind specific cellular structures and have been derivatised with a fluorescent reporter. A major example of this class of fluorescent stain is phalloidin, which is used to stain actin fibers in mammalian cells. A new peptide, known as the Collagen Hybridizing Peptide, can also be conjugated with fluorophores and used to stain "denatured" collagen fibers. Staining of the plant cell walls is performed using stains or dyes that bind cellulose or pectin. The quest for fluorescent probes with a high specificity that also allow live imaging of plant cells is ongoing.  There are many fluorescent molecules called fluorophores or fluorochromes such as fluorescein, Alexa Fluors, or DyLight 488, which can be chemically linked to a different molecule which binds the target of interest within the sample.

Immunofluorescence.

Immunofluorescence is a technique which uses the highly specific binding of an antibody to its antigen in order to label specific proteins or other molecules within the cell. A sample is treated with a primary antibody specific for the molecule of interest. A fluorophore can be directly conjugated to the primary antibody. Alternatively a secondary antibody, conjugated to a fluorophore, which binds specifically to the first antibody can be used. For example, a primary antibody raised in a mouse which recognises tubulin combined with a secondary anti-mouse antibody derivatised with a fluorophore could be used to label microtubules in a cell.

Fluorescent proteins.

The modern understanding of genetics and the techniques available for modifying DNA allow scientists to genetically modify proteins to also carry a fluorescent protein reporter. In biological samples this allows a scientist to directly make a protein of interest fluorescent. The protein location can then be directly tracked, including in live cells.

Limitations.

Fluorophores lose their ability to fluoresce as they are illuminated in a process called photobleaching. Photobleaching occurs as the fluorescent molecules accumulate chemical damage from the electrons excited during fluorescence. Photobleaching can severely limit the time over which a sample can be observed by fluorescence microscopy. Several techniques exist to reduce photobleaching such as the use of more robust fluorophores, by minimizing illumination, or by using photoprotective scavenger chemicals.  Fluorescence microscopy with fluorescent reporter proteins has enabled analysis of live cells by fluorescence microscopy, however cells are susceptible to phototoxicity, particularly with short wavelength light. Furthermore, fluorescent molecules have a tendency to generate reactive chemical species when under illumination which enhances the phototoxic effect.  Unlike transmitted and reflected light microscopy techniques fluorescence microscopy only allows observation of the specific structures which have been labeled for fluorescence. For example, observing a tissue sample prepared with a fluorescent DNA stain by fluorescence microscopy only reveals the organization of the DNA within the cells and reveals nothing else about the cell morphologies.

Sub-diffraction techniques.

The wave nature of light limits the size of the spot to which light can be focused due to the diffraction limit. This limitation was described in the 19th century by Ernst Abbe and "limits an optical microscope's resolution to approximately half of the wavelength of the light used." Fluorescence microscopy is central to many techniques which aim to reach past this limit by specialized optical configurations.  Several improvements in microscopy techniques have been invented in the 20th century and have resulted in increased resolution and contrast to some extent. However they did not overcome the diffraction limit. In 1978 first theoretical ideas have been developed to break this barrier by using a 4Pi microscope as a confocal laser scanning fluorescence microscope where the light is focused ideally from all sides to a common focus which is used to scan the object by 'point-by-point' excitation combined with 'point-by-point' detection. However, the first experimental demonstration of the 4pi microscope took place in 1994. 4Pi microscopy maximizes the amount of available focusing directions by using two opposing objective lenses or two-photon excitation microscopy using redshifted light and multi-photon excitation.  Integrated correlative microscopy combines a fluorescence microscope with an electron microscope. This allows one to visualize ultrastructure and contextual information with the electron microscope while using the data from the fluorescence microscope as a labelling tool.  The first technique to really achieve a sub-diffraction resolution was STED microscopy, proposed in 1994. This method and all techniques following the RESOLFT concept rely on a strong non-linear interaction between light and fluorescing molecules. The molecules are driven strongly between distinguishable molecular states at each specific location, so that finally light can be emitted at only a small fraction of space, hence an increased resolution.  As well in the 1990s another super resolution microscopy method based on wide field microscopy has been developed. Substantially improved size resolution of cellular nanostructures stained with a fluorescent marker was achieved by development of SPDM localization microscopy and the structured laser illumination (spatially modulated illumination, SMI). Combining the principle of SPDM with SMI resulted in the development of the Vertico SMI microscope. Single molecule detection of normal blinking fluorescent dyes like green fluorescent protein (GFP) can be achieved by using a further development of SPDM the so-called SPDMphymod technology which makes it possible to detect and count two different fluorescent molecule types at the molecular level (this technology is referred to as two-color localization microscopy or 2CLM).  Alternatively, the advent of photoactivated localization microscopy could achieve similar results by relying on blinking or switching of single molecules, where the fraction of fluorescing molecules is very small at each time. This stochastic response of molecules on the applied light corresponds also to a highly nonlinear interaction, leading to subdiffraction resolution.

A fluorescence microscope is an optical microscope that uses fluorescence instead of, or in addition to, scattering, reflection, and attenuation or absorption, to study the properties of organic or inorganic substances. "Fluorescence microscope" refers to any microscope that uses fluorescence to generate an image, whether it is a more simple set up like an epifluorescence microscope or a more complicated design such as a confocal microscope, which uses optical sectioning to get better resolution of the fluorescence image.

La microscopie en fluorescence (ou en "épifluorescence") est une technique utilisant un microscope optique en tirant profit du phénomène de fluorescence et de phosphorescence, au lieu de, ou en plus de l'observation classique par réflexion ou absorption de la lumière visible naturelle ou artificielle. On peut ainsi observer divers objets, substances (organiques ou inorganiques) ou échantillons d'organismes morts ou vivants.


History.

The start of telephony in Belgium.

In 1879, the Belgian telegraph service installed a telephone line at the Parliament. That same year, several private contractors submitted requests to operate the telephone networks in various Belgian cities. The lack of legislation during the first few years of operation reduced the telephone network's chances of developing. It also forced the Belgian authorities to develop a legislative framework to regulate the operation of telephony in Belgium.  In 1896, the whole telephony sector passed into the hands of a public company.  In 1913, a large portion of Belgium was accessible by telephone. Although the number of subscribers was still small, the majority of railway stations, post offices and telegraphs were equipped with public telephone booths.

After World War I: the shift to an autonomous public-sector company.

World War I had caused a complete and utter suspension of telecommunications in Belgium. One of the reasons for this was the financial dependency of the public company. The damage caused during the war and the dismantling of parts of the networks meant that colossal investments were needed to manage the telegraphs and telephones.  The national telegraph and telephone company (Régie des Télégraphes et Téléphones, "RTT") was set up on 19 July 1930. The public-sector company thus became autonomous: it was no longer dependent on annual State budgets and was authorized to carry out its own management.

Inclusion of the RTT in the state industrial policy.

When the RTT was created, huge sums were invested in the Belgian telephone network. More and more sectors of society now had access to telephony. At the same time, another development was unfolding that would quickly entail a major expense for the company.  During the economic crisis of the 1930s, the State would involve the RTT in its industrial and employment policy. In an attempt to reduce the high unemployment rate in the sector the State forced the entire Belgian telephone network to become automated. This development strongly restricted the autonomy of the RTT. Although the law of 1930 explicitly stated that the company could draw up and implement an investment plan in an autonomous manner, by imposing its employment policy, the State was going against the basic principle of the law. After the war, this would soon become a structural problem for the RTT.

From a leading-edge company to crisis: the RTT in the post-war period.

After World War II, the RTT was faced with considerable damage and the dismantling of part of its networks. To give a quick boost to the sector, the State decided to intervene financially.  During this period, the demand for telecommunication services increased at an alarming rate. The number of subscribers quickly rose from approximately 350,000 in 1946 to 522,000 in 1951 and 1,049,000 in 1965. This growth in the customer base led to a very high rate of investment. Thanks to this, towards the end of the 1960s, the RTT found itself at the forefront of social and technological development.  But this expansionist approach had a negative side. From the late 1960s, the losses began to accumulate. And the global economic crisis in 1973 did not help matters: the company's financial situation only worsened. This situation would lead the RTT to carry out major reorganization programs during the mid-1970s.  During the 1980s, it became clear that the telecommunications sector would become one of the key development poles at the end of the 20th century. Thus, in 1981, the RTT management began a major reorganization to solve certain structural problems within the company.  During that period, another player entered the scene. In 1987, the European Commission published its Green Paper on telecommunications, which focused on liberalization.

The 1990s: the Belgacom law and the evolution of the sector under European influence.

The Green Paper of 1987 was incorporated into the Belgian law of 21 March 1991, which created a new type of public-sector company with greater management autonomy. The Belgian telecommunications sector was thus reorganized and saw the creation of Belgacom, an autonomous public-sector company. This law aims to create an environment that is favorable to the competitive development of the telecommunications market in Belgium. From now on, a management contract had to define the prerogatives of the company and the public authorities in order to guarantee the offer of a certain number of general utility public services and a wider management autonomy than that provided for in the law of 1930.  In 1994, the European convergence process began to accelerate. In a new Green Paper, the European Commission declared that the operation of the networks and telephony must also be open to competition. 1994 was also the year in which Belgacom founded Proximus, Belgium's first mobile network.  On 1 July 1994, this activity, as well as the old analog Mob2 system, was transferred to a subsidiary, Belgacom Mobile, with the following shareholding: 75% Belgacom – 25% Air Touch, then Vodafone in 1999.  At the same time, Belgacom faced competition by forming partnerships with Ameritech, Tele Danmark and Singapore Telecom. Various Belgian financial institutions responded by forming a consortium, which was called ADSB. The Belgian State holds 50.1% of the shares and therefore remains the main shareholder.  The BeST plan, which was mainly aimed at restructuring the company and dividing it into four "business units," was implemented in 2001. Belgacom also disposed of certain activities such as Belgacom France, Ben, its security activities as well as the French activities of Infosources.  The human dimension of the BeST plan was implemented in the course of 2002. The company, which employed too many staff at the time, had many reasons for adopting this plan: voluntary offers for departure, part-time work and reconversion were proposed to a large portion of the staff.  In an increasingly open market, in which competition is more aggressive by the day, Belgacom decided to bet on the future by radically changing its image in 2003. A change of logo, colors and the promise to be closer to its customers were the bases on which the former RTT began to work.  These radical changes in the company's philosophy were followed by the operator's initial public offering. On 22 March 2004, Belgacom was listed for the first time on the Euronext stock market. The Belgian State remains the majority shareholder with 50% + 1 of all shares, while the ADSB consortium sold all its shares.  This enabled the Belgian incumbent operator to free considerable sums to finance its objectives. Indeed, the time has come for broadband, and the funding of the Broadway project (to cover the whole Belgian territory with fiber optic cables) requires huge investments.  2004 was also the year in which the incumbent operator carried out its first digital TV tests with a view to finding new sources of income in an increasingly competitive market.

From 2005 to 2010: consolidation, convergence and first bundled offers.

2005 was marked by two important events for Belgacom: the launch of Belgacom TV and the takeover bid on Telindus. The first signs of digital TV in Belgium appeared in 2004, when Belgacom started its first digital TV tests in a few hundred homes. In May 2005, the Belgian operator took the market by surprise when it acquired the rights to broadcast professional Belgian football (D1 & D2) for the next three seasons through its subsidiary Skynet iMotions Activities.  This step anticipated the imminent launch of Belgacom TV in June 2005. This digital TV offer via ADSL was the first of its kind in Belgium and transformed Belgacom into a quadruple player, offering fixed telephony, mobile telephony, high-speed Internet as well as television. It also enabled the Belgian company to secure new sources of income, given that the profit margins on its traditional activities are becoming increasingly reduced.  2005 was also the year of the takeover bid on Telindus, a leader in the network integration sector. An initial bid, considered "hostile" by the Telindus management, was made in September. This marked the start of a stock-market saga that would go on for almost four months. Tensions were high between the two companies, and the different stakeholders fought it out in the press. After a counter bid launched by France Télécom, Belgacom eventually carried the day, arriving at a conditional partnership agreement with Telindus in late December. Belgacom had responded by increasing its bid for Telindus, which enabled the conditional partnership agreement to be reached.  In August 2006 Belgacom the acquisition of Vodafone’s 25% share in Proximus, giving the firm a wholly owned quadruple play offering. The firm subsequently launched its first bundled offers between Belgacom and Proximus.  At the end of March 2009, the company had more than 555,000 Belgacom TV customers. The coverage rate of digital television increased to 80% of the population.  On 26 June 2008, Belgacom announced an agreement to acquire Tele2's Luxembourg division (including the Tango cellular network in Luxembourg and Liechtenstein). Later the same year Belgacom completed the acquisition of broadband provider Scarlet for €175 million. The Belgian competition authority allowed the deal to go ahead after Belgacom agreed to some divestments, including Scarlet's fibre network.  On 4 January 2010, Belgacom Group completes the full integration of its subsidiaries Belgacom Mobile SA/Proximus, Telindus NV, Telindus Sourcing SA, the activities of Belgacom Skynet SA and the Belgian activities of Telindus Group NV into Belgacom SA to form one legal entity. The other subsidiaries remain separate legal entities.  On 4 September 2010, Didier Bellens announced the future launch of a second platform (called 'platform n° 2' for the time being) next to Belgacom TV. This platform will focus on interactivity and online gaming services.

Main brands and activities of the group.

Proximus NV/SA.

The Belgacom brand was established in 1992, following the dissolution of the RTT. As the company changed its articles of association several times to comply with national and European legal provisions, its logo changed with it. The very first logo, with the inverted orange arrows, was replaced by a more "human" logo. On 4 January 2010, Belgacom NV/SA absorbed the activities of mobile operator Proximus and ICT services provider Telindus, thus ending their existence as subsidiaries.

Proximus Mobile (Belgacom Mobile).

The Proximus brand was created in 1994, following the launch of the first mobile network. In 2005, an Interbrand study found Proximus to be the second brand in Belgium.  Proximus offers a wide line of products and mobile telecommunications services to its residential and professional customers, therefore having the largest HSDPA coverage in Belgium. In 2010, Belgacom ended the existence of Belgacom Mobile as a separate legal entity and absorbed all of its activities into Belgacom SA.

Proximus TV (Belgacom TV).

Proximus TV is the brand of the digital TV offer in Belgium, launched by Belgacom in the summer of 2005. It was the first offer of its kind in the country. Proximus TV currently holds 18% market share of digital TV in Belgium. In the Summer of 2011, Proximus launched a new service called 'TV Overal' (in Dutch) or 'TV Partout' (in French). People can download this free application from the Android Market or the AppStore. It lets you watch TV on your Smartphone or Tablet over a mobile network connection (with 1 hour free per day for Proximus Internet Everywhere customers) or over a WiFi connection.

Scarlet.

Scarlet was founded in the Netherlands in 1992. The brand aims to offer its customers basic low-cost products in the areas of fixed and mobile telephony and the internet.

Skynet.

Skynet was founded in 1995. At the time, it was one of the first commercial Internet providers in Belgium. From 2005, following Belgacom's takeover of the Group's Internet activities, the Skynet brand continued to exist but only in association with the Group's Web content activities.

Tango.

Tango is the company under which the Belgacom Group markets its offers in Luxembourg and Liechtenstein. The company, which was launched in 1998, has become the second-largest mobile player on the Luxembourg market.

Telindus (Belgacom ICT).

Telindus was created in June 2006, following the purchase of Telindus by Belgacom. Since then, the ICT activities of the Belgacom Group have been offered under the Telindus brand, which accordingly changed its logo and brand style. The new Telindus brand was inspired by the changes Telindus has undergone since it was first founded, i.e. from being a technology supplier to a solution, integrator and sourcing partner. In 2010, Belgacom ended the existence of Telindus as a separate legal entity and absorbed all of its activities into Belgacom SA.

BICS (Belgacom International Carrier Services).

BICS (Belgacom International Carrier Services) was founded in 2005 and is a joint venture between Belgacom, Swisscom Fixnet and MTN Group. The company operates in the telecommunication industry and is responsible for international carrier activities. It provides wholesale carrier services to wireline and wireless operators, and service providers globally.

Other companies and activities within the group.

PingPing.

PingPing is the neutral, independent brand of Mobile-for grouping all the mobile micropayment activities.

Euremis.

The Euremis brand represents the experience of Belgacom Group in CRM mobile solutions. Under the Euremis brand, Belgacom offers mobile customer management solutions to sales staff in the FMCG (Fast-Moving Consumer Goods) and pharmaceutical sectors. Euremis was founded as an independent company in 2002 and acquired in September 2006 by Proximus. On 23 August 2010 Belgacom Group announced to end the existence of Euremis as a separate legal entity and fully absorb its activities into Belgacom SA, as of 1 October 2010.

Group structure.

In 2007, the Belgacom Group reviewed the entire organisation and created a new operating structure based on 4 pillars:

Consumer Business Unit (CBU).

The Consumer Business Unit markets voice products and services, internet and television, both on fixed and mobile networks, for residential clients.

Enterprise Business Unit (EBU).

The Enterprise Business Unit meets the ICT needs of professional clients.

Service Delivery Engine & Wholesale (SDE&W).

Service Delivery Engine and Wholesale (SDE&W) groups together the network and IT services. Its wholesale activity offers telecommunications services to other operators and suppliers on the Belgian market.

Staff and Support (S&S).

This unit brings together all the horizontal functions that support the Group's activities.

Shareholding structure.

Situation as of 31 January 2011:

Financial data.

Financial data in millions of euros:

GCHQ hack.

Material from whistleblower Edward Snowden's leak in 2013 indicated that the British intelligence service GCHQ directed a cyber attack code named Operation Socialist against the computer network of Belgacom using Regin malware. While Belgacom stated the attack had limited success and no customer data was compromised, published materials state that GCHQ compromised "both deep into the network and at the edge of the network" and gained access to Belgacom customers' data streams.

The Proximus Group (previously known as Belgacom Group) is the largest telecommunications company in Belgium, headquartered in Brussels. Proximus Group is primarily state owned, with the Belgian state holding 53.3% + 1 share. Proximus Group offerings include fixed line and mobile communications through the Proximus brand and ICT services to the professional market under the Telindus brand.

Le groupe Belgacom-Proximus, composé de Proximus SA (anciennement Belgacom S.A.) et de ses filiales (Belgacom ICS, Telindus, Belgacom Skynet, Scarlet, Tango), est une entreprise belge de télécommunications, la première entreprise de télécommunications en Belgique, issue de l'opérateur historique national.


Length.

Egyptian units of length are attested from the Early Dynastic Period. Although it dates to the 5th dynasty, the Palermo stone recorded the level of the Nile River during the reign of the Early Dynastic pharaoh Djer, when the height of the Nile was recorded as 6 cubits and 1 palm (about ). A 3rd-dynasty diagram shows how to construct an elliptical vault using simple measures along an arc. The ostracon depicting this diagram was found near the Step Pyramid of Saqqara. A curve is divided into five sections and the height of the curve is given in cubits, palms, and digits in each of the sections. At some point, lengths were standardized by cubit rods. Examples have been found in the tombs of officials, noting lengths up to remen. Royal cubits were used for land measures such as roads and fields. Fourteen rods, including one double-cubit rod, were described and compared by Lepsius. Two examples are known from the Saqqara tomb of Maya, the treasurer of Tutankhamun. Another was found in the tomb of Kha (TT8) in Thebes. These cubits are about long and are divided into palms and hands: each palm is divided into four fingers from left to right and the fingers are further subdivided into ro from right to left. The rules are also divided into hands so that for example one foot is given as three hands and fifteen fingers and also as four palms and sixteen fingers.  Surveying and itinerant measurement were undertaken using rods, poles, and knotted cords of rope. A scene in the tomb of Menna in Thebes shows surveyors measuring a plot of land using rope with knots tied at regular intervals. Similar scenes can be found in the tombs of Amenhotep-Sesi, Khaemhat and Djeserkareseneb. The balls of rope are also shown in New Kingdom statues of officials such as Senenmut, Amenemhet-Surer, and Penanhor.  The digit was also subdivided into smaller fractions of,,, and. Minor units include the Middle Kingdom reed of 2 royal cubits, the Ptolemaic xylon (,."timber") of three royal cubits, the Ptolemaic fathom (, "orgyiá"; ;, "hpot") of four lesser cubits, and the kalamos of six royal cubits.

Area.

Records of land area also date to the Early Dynastic Period. The Palermo stone records grants of land expressed in terms of "kha" and "setat". Mathematical papyri also include units of land area in their problems. For example, several problems in the Moscow Mathematical Papyrus give the area of rectangular plots of land in terms of "setat" and the ratio of the sides and then require the scribe to solve for their exact lengths.  The "setat" was the basic unit of land measure and may originally have varied in size across Egypt's nomes. Later, it was equal to one square "khet", where a "khet" measured 100 "cubits". The "setat" could be divided into strips one "khet" long and ten "cubit" wide (a "kha").  During the Old Kingdom:  During the Middle and New Kingdom, the "eighth", "fourth", "half", and "thousand" units were taken to refer to the "setat" rather than the cubit strip:  During the Ptolemaic period, the cubit strip square was surveyed using a length of 96 cubits rather than 100, although the "aroura" was still figured to compose 2,756.25m. A 36squarecubit area was known as a "kalamos" and a 144squarecubit area as a "hamma". The uncommon "bikos" may have been "hammata" or another name for the cubit strip. The Coptic "shipa" () was a land unit of uncertain value, possibly derived from Nubia.

Volume.

Units of volume appear in the mathematical papyri. For example, computing the volume of a circular granary in RMP42 involves cubic cubits, khar, heqats, and quadruple heqats. RMP80 divides heqats of grain into smaller henu.  The oipe was also formerly romanized as the "apet".

Weight.

Weights were measured in terms of deben. This unit would have been equivalent to 13.6 grams in the Old Kingdom and Middle Kingdom. During the New Kingdom however it was equivalent to 91 grams. For smaller amounts the qedet ( of a deben) and the shematy ( of a deben) were used.  The qedet or kedet is also often known as the "kite", from the Coptic form of the same name ( or ). In 19th-century sources, the deben and qedet are often mistakenly transliterated as the "uten" and "kat" respectively, although this was corrected by the 20th century.

Time.

The former annual flooding of the Nile organized prehistoric and ancient Egypt into three seasons: Akhet ("Flood"), Peret ("Growth"), and Shemu or Shomu ("Low Water" or "Harvest").  The Egyptian civil calendar in place by Dynasty V followed regnal eras resetting with the ascension of each new pharaoh. It was based on the solar year and apparently initiated during a heliacal rising of Sirius following a recognition of its rough correlation with the onset of the Nile flood. It followed none of these consistently, however. Its year was divided into 3 seasons, 12 months, 36 decans, or 360 days with another 5 epagomenal days—celebrated as the birthdays of five major gods but feared for their ill luck—added "upon the year". The Egyptian months were originally simply numbered within each season but, in later sources, they acquired names from the year's major festivals and the three decans of each one were distinguished as "first", "middle", and "last". It has been suggested that during the Nineteenth Dynasty and the Twentieth Dynasty the last two days of each decan were usually treated as a kind of weekend for the royal craftsmen, with royal artisans free from work. This scheme lacked any provision for leap year intercalation until the introduction of the Alexandrian calendar by Augustus in the 20s, causing it to slowly move through the Sothic cycle against the solar, Sothic, and Julian years. Dates were typically given in a YMD format.  The civil calendar was apparently preceded by an observational lunar calendar which was eventually made lunisolar and fixed to the civil calendar, probably in 357. The months of these calendars were known as "temple months" and used for liturgical purposes until the closing of Egypt's pagan temples under Theodosius I in the 390s and the subsequent suppression of individual worship by his successors.  Smaller units of time were vague approximations for most of Egyptian history. Hours—known by a variant of the word for "stars"—were initially only demarcated at night and varied in length. They were measured using decan stars and by water clocks. Equal 24-part divisions of the day were only introduced in 127. Division of these hours into 60 equal minutes is attested in Ptolemy's 2nd-century works.

The ancient Egyptian units of measurement are those used by the dynasties of ancient Egypt prior to its incorporation in the Roman Empire and general adoption of Roman, Greek, and Byzantine units of measurement. The units of length seem to have originally been anthropic, based on various parts of the human body, although these were standardized using cubit rods, strands of rope, and official measures maintained at some temples.

Les unités de mesure dans l'Égypte antique nous proviennent de la grande quantité de documents égyptiens d'ordre comptable, qui nous sont parvenus. Rien d'étonnant pour cette civilisation bureaucratique ou les scribes avaient comme tâche principale de comptabiliser les récoltes (pour calculer les impôts) et de refaire l'arpentage et le bornage des terres agricoles que la crue du Nil effaçait chaque année.


Medical uses.

Antibiotics are used to treat or prevent bacterial infections, and sometimes protozoan infections. (Metronidazole is effective against a number of parasitic diseases). When an infection is suspected of being responsible for an illness but the responsible pathogen has not been identified, an empiric therapy is adopted. This involves the administration of a broad-spectrum antibiotic based on the signs and symptoms presented and is initiated pending laboratory results that can take several days.  When the responsible pathogenic microorganism is already known or has been identified, definitive therapy can be started. This will usually involve the use of a narrow-spectrum antibiotic. The choice of antibiotic given will also be based on its cost. Identification is critically important as it can reduce the cost and toxicity of the antibiotic therapy and also reduce the possibility of the emergence of antimicrobial resistance. To avoid surgery, antibiotics may be given for non-complicated acute appendicitis.  Antibiotics may be given as a preventive measure and this is usually limited to at-risk populations such as those with a weakened immune system (particularly in HIV cases to prevent pneumonia), those taking immunosuppressive drugs, cancer patients, and those having surgery. Their use in surgical procedures is to help prevent infection of incisions. They have an important role in dental antibiotic prophylaxis where their use may prevent bacteremia and consequent infective endocarditis. Antibiotics are also used to prevent infection in cases of neutropenia particularly cancer-related.

Administration.

There are many different routes of administration for antibiotic treatment. Antibiotics are usually taken by mouth. In more severe cases, particularly deep-seated systemic infections, antibiotics can be given intravenously or by injection. Where the site of infection is easily accessed, antibiotics may be given topically in the form of eye drops onto the conjunctiva for conjunctivitis or ear drops for ear infections and acute cases of swimmer's ear. Topical use is also one of the treatment options for some skin conditions including acne and cellulitis.

Prevalence.

Antibiotic consumption varies widely between countries. The WHO report on surveillance of antibiotic consumption’ published in 2018 analysed 2015 data from 65 countries. As

Side effects.

Antibiotics are screened for any negative effects before their approval for clinical use, and are usually considered safe and well tolerated. However, some antibiotics have been associated with a wide extent of adverse side effects ranging from mild to very severe depending on the type of antibiotic used, the microbes targeted, and the individual patient. Side effects may reflect the pharmacological or toxicological properties of the antibiotic or may involve hypersensitivity or allergic reactions. Adverse effects range from fever and nausea to major allergic reactions, including photodermatitis and anaphylaxis. Safety profiles of newer drugs are often not as well established as for those that have a long history of use.  Common side-effects include diarrhea, resulting from disruption of the species composition in the intestinal flora, resulting, for example, in overgrowth of pathogenic bacteria, such as "Clostridium difficile". Antibacterials can also affect the vaginal flora, and may lead to overgrowth of yeast species of the genus "Candida" in the vulvo-vaginal area. Additional side effects can result from interaction with other drugs, such as the possibility of tendon damage from the administration of a quinolone antibiotic with a systemic corticosteroid.  Some antibiotics may also damage the mitochondrion, a bacteria-derived organelle found in eukaryotic, including human, cells. Mitochondrial damage cause oxidative stress in cells and has been suggested as a mechanism for side effects from fluoroquinolones. They are also known to affect chloroplasts.

Correlation with obesity.

Exposure to antibiotics early in life is associated with increased body mass in humans and mouse models. Early life is a critical period for the establishment of the intestinal microbiota and for metabolic development. Mice exposed to subtherapeutic antibiotic treatment – with either penicillin, vancomycin, or chlortetracycline had altered composition of the gut microbiota as well as its metabolic capabilities. One study has reported that mice given low-dose penicillin (1 μg/g body weight) around birth and throughout the weaning process had an increased body mass and fat mass, accelerated growth, and increased hepatic expression of genes

Interactions.

Birth control pills.

There are few well-controlled studies on whether antibiotic use increases the risk of oral contraceptive failure. The majority of studies indicate antibiotics do not interfere with birth control pills, such as clinical studies that suggest the failure rate of contraceptive pills caused by antibiotics is very low (about 1%). Situations that may increase the risk of oral contraceptive failure include non-compliance (missing taking the pill), vomiting, or diarrhea. Gastrointestinal disorders or interpatient variability in oral contraceptive absorption affecting ethinylestradiol serum levels in the blood. Women with menstrual irregularities may be at higher risk of failure and should be advised to use backup contraception during antibiotic

Alcohol.

Interactions between alcohol and certain antibiotics may occur and may cause side effects and decreased effectiveness of antibiotic therapy. While moderate alcohol consumption is unlikely to interfere with many common antibiotics, there are specific types of antibiotics, with which alcohol consumption may cause serious side effects. Therefore, potential risks of side effects and

Pharmacodynamics.

The successful outcome of antimicrobial therapy with antibacterial compounds depends on several factors. These include host defense mechanisms, the location of infection, and the pharmacokinetic and pharmacodynamic properties of the antibacterial. A bactericidal activity of antibacterials may depend on the bacterial growth phase, and it often requires ongoing metabolic activity and division of bacterial cells. These findings are based on laboratory studies, and in clinical settings have also been shown to eliminate bacterial infection. Since the activity of antibacterials depends frequently on its concentration, "in vitro" characterization of antibacterial activity commonly includes the determination of the minimum inhibitory concentration and minimum bactericidal concentration of an antibacterial. To predict clinical outcome, the antimicrobial activity of an antibacterial is usually combined with its pharmacokinetic profile, and several pharmacological parameters are used as markers of drug efficacy.

Combination therapy.

In important infectious diseases, including tuberculosis, combination therapy (i.e., the concurrent application of two or more antibiotics) has been used to delay or prevent the emergence of resistance. In acute bacterial infections, antibiotics as part of combination therapy are prescribed for their synergistic effects to improve treatment outcome as the combined effect of both antibiotics is

Classes.

Antibiotics are commonly classified based on their mechanism of action, chemical structure, or spectrum of activity. Most target bacterial functions or growth processes. Those that target the bacterial cell wall (penicillins and cephalosporins) or the cell membrane (polymyxins), or interfere with essential bacterial enzymes (rifamycins, lipiarmycins, quinolones, and sulfonamides) have bactericidal activities. Protein synthesis inhibitors (macrolides, lincosamides, and

Production.

With advances in medicinal chemistry, most modern antibacterials are semisynthetic modifications of various natural compounds. These include, for example, the beta-lactam antibiotics, which include the penicillins (produced by fungi in the genus "Penicillium"), the cephalosporins, and the carbapenems. Compounds that are still isolated from living organisms are the aminoglycosides, whereas other antibacterials—for example, the sulfonamides,

Resistance.

The emergence of resistance of bacteria to antibiotics is a common phenomenon. Emergence of resistance often reflects evolutionary processes that take place during antibiotic therapy. The antibiotic treatment may select for bacterial strains with physiologically or genetically enhanced capacity to survive high doses of antibiotics. Under certain conditions, it may result in preferential growth of resistant bacteria, while growth of susceptible bacteria is inhibited by the drug. For example, antibacterial selection for strains having previously acquired antibacterial-resistance genes was demonstrated in 1943 by the Luria–Delbrück experiment. Antibiotics such as penicillin and erythromycin, which used to have a high efficacy against many bacterial species and strains, have become less effective, due to the increased resistance of many bacterial strains.  Resistance may take the form of biodegredation of pharmaceuticals, such as sulfamethazine-degrading soil bacteria introduced to sulfamethazine through medicated pig feces. The survival of bacteria often results from an inheritable resistance, but the growth of resistance to antibacterials also occurs through horizontal gene transfer. Horizontal transfer is more likely to happen in locations of frequent antibiotic use.  Antibacterial resistance may impose a biological cost, thereby reducing fitness of resistant strains, which can limit the spread of antibacterial-resistant bacteria, for example, in the absence of antibacterial compounds. Additional mutations, however, may compensate for this fitness cost and can aid the survival of these bacteria.  Paleontological data show that both antibiotics and antibiotic resistance are ancient compounds and mechanisms. Useful antibiotic targets are those for which mutations negatively impact bacterial reproduction or viability.  Several molecular mechanisms of antibacterial resistance exist. Intrinsic antibacterial resistance may be part of the genetic makeup of bacterial strains. For example, an antibiotic target may be absent from the bacterial genome. Acquired resistance results from a mutation in the bacterial chromosome or the acquisition of extra-chromosomal DNA. Antibacterial-producing bacteria have evolved resistance mechanisms that have been shown to be similar to, and may have been transferred to, antibacterial-resistant strains. The spread of antibacterial resistance often occurs through vertical transmission of mutations during growth and by genetic recombination of DNA by horizontal genetic exchange. For instance, antibacterial resistance genes can be exchanged between different bacterial strains or species via plasmids that carry these resistance genes. Plasmids that carry several different resistance genes can confer resistance to multiple antibacterials. Cross-resistance to several antibacterials may also occur when a resistance mechanism encoded by a single gene conveys resistance to more than one antibacterial compound.  Antibacterial-resistant strains and species, sometimes referred to as "superbugs", now contribute to the emergence of diseases that were for a while well controlled. For example, emergent bacterial strains causing tuberculosis that are resistant to previously effective antibacterial treatments pose many therapeutic challenges. Every year, nearly half a million new cases of multidrug-resistant tuberculosis (MDR-TB) are estimated to occur worldwide. For example, NDM-1 is a newly identified enzyme conveying bacterial resistance to a broad range of beta-lactam antibacterials. The United Kingdom's Health Protection Agency has stated that "most isolates with NDM-1 enzyme are resistant to all standard intravenous antibiotics for treatment of severe infections." On 26 May 2016 an E coli bacteria "superbug" was identified in the United States resistant to colistin, "the last line of defence" antibiotic.

History.

Before the early 20th century, treatments for infections were based primarily on medicinal folklore. Mixtures with antimicrobial properties that were used in treatments of infections were described over 2,000 years ago. Many ancient cultures, including the ancient Egyptians and ancient Greeks, used specially selected mold and plant materials to treat infections. Nubian mummies studied in the 1990s were found to contain significant levels of Tetracycline. The beer brewed at that time was conjectured to have been the source. The use of antibiotics in modern medicine began with the discovery of synthetic antibiotics derived from dyes.

Synthetic antibiotics derived from dyes.

Synthetic antibiotic chemotherapy as a science and development of antibacterials began in Germany with Paul Ehrlich in the late 1880s. Ehrlich noted certain dyes would color human, animal, or bacterial cells, whereas others did not. He then proposed the idea that it might be possible to create chemicals that would act as a selective drug that would bind to and kill bacteria without harming the human host. After screening hundreds of dyes against various organisms, in 1907, he discovered a medicinally useful drug, the first synthetic antibacterial organoarsenic compound salvarsan, now called arsphenamine.  This heralded the era of antibacterial treatment that was begun with the discovery of a series of arsenic-derived synthetic antibiotics by both Alfred Bertheim and Ehrlich in 1907. Ehrlich and Bertheim had experimented with various chemicals derived from dyes to treat trypanosomiasis in mice and spirochaeta infection in rabbits. While their early compounds were too toxic, Ehrlich and Sahachiro

Penicillin and other natural antibiotics.

Observations about the growth of some microorganisms inhibiting the growth of other microorganisms have been reported since the late 19th century. These observations of antibiosis between microorganisms led to the discovery of natural antibacterials. Louis Pasteur observed, "if we could intervene in the antagonism observed between some bacteria, it would offer perhaps the greatest hopes for therapeutics".  In 1874, physician Sir William Roberts noted that cultures of the mold "Penicillium glaucum" that is used in the making of some types of blue cheese did not display bacterial contamination. In 1876, physicist John Tyndall also contributed to this field. Pasteur conducted research showing that "Bacillus anthracis" would not

Late 20th century.

During the mid-20th century, the number of new antibiotic substances introduced for medical use increased significantly. From 1935

Etymology.

The term 'antibiosis', meaning "against life", was introduced by the French bacteriologist Jean Paul Vuillemin as a descriptive name of the phenomenon exhibited by these early antibacterial drugs. Antibiosis was first described in 1877 in bacteria when Louis Pasteur and Robert Koch observed that an airborne bacillus could inhibit the growth of "Bacillus anthracis". These drugs were later renamed antibiotics by Selman Waksman, an American microbiologist, in 1942.  The term "antibiotic" was first used in 1942 by Selman Waksman and his collaborators in journal articles to describe any

Research.

Alternatives.

The increase in bacterial strains that are resistant to conventional antibacterial therapies together with decreasing number of new antibiotics currently being developed in the drug pipeline has prompted the

Resistance and modifying agents.

One strategy to address bacterial drug resistance is the discovery and application of compounds that modify resistance to common antibacterials. Resistance modifying agents are capable of partly or completely suppressing bacterial resistance

Vaccines.

Vaccines rely on immune modulation or augmentation. Vaccination either excites or reinforces the immune competence of a host to ward off infection, leading to the activation of macrophages, the production of antibodies, inflammation, and other classic

Phage therapy.

Phage therapy is another method for treating antibiotic-resistant strains of bacteria. Phage therapy infects pathogenic bacteria with their own viruses. Bacteriophages and their host ranges are extremely specific for certain bacteria, thus, unlike antibiotics, they do not disturb the host organism and intestinal microflora. Bacteriophages, also known simply as phages, infect and can kill bacteria and affect bacterial growth primarily during lytic cycles. Phages insert their DNA into the bacterium, where it is transcribed and used to make new phages, after which the cell will lyse, releasing new phage that are able to infect

Phytochemicals.

Plants are an important source of antimicrobial compounds and traditional healers have long used plants to prevent or cure infectious diseases. There is a recent renewed interest into the use of natural products for the identification of new members of the 'antibiotic-ome' (defined as natural products with antibiotic activity), and their application in antibacterial

New antibiotics development.

Both the WHO and the Infectious Disease Society of America reported that the weak antibiotic pipeline does not match bacteria's increasing ability to develop resistance. The Infectious Disease Society of America report noted that the number of new antibiotics approved for marketing per year had been declining and identified seven antibiotics against the Gram-negative bacilli currently in phase 2 or phase 3 clinical trials. However, these drugs did not address the entire spectrum of resistance of Gram-negative bacilli. According to the WHO fifty one new therapeutic entities - antibiotics (including combinations), are in phase 1-3 clinical trials as of May 2017. Recent entries in the clinical pipeline targeting multidrug-resistant Gram-positive pathogens has improved the treatment options due to marketing approval of new antibiotic classes, the oxazolidinones and cyclic lipopeptides. However, resistance to these antibiotics is certainly likely to occur, the need for the development new antibiotics against those pathogens still remains a high priority. Recent drugs in development that target Gram-negative bacteria have focused on re-working existing drugs to target specific microorganisms or specific types of resistance.  A few antibiotics have received marketing authorization in the last seven years. The cephalosporin ceftaroline and the lipoglycopeptides oritavancin and

An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention of such infections. They may either kill or inhibit the growth of bacteria. A limited number of antibiotics also possess antiprotozoal activity. Antibiotics are not effective against viruses such as the common cold or influenza ; drugs which inhibit viruses are termed antiviral drugs or antivirals rather than antibiotics.

Un antibiotique (du grec "anti" : « contre », et "bios" : « la vie ») est une substance naturelle ou synthétique qui détruit ou bloque la croissance des bactéries. Dans le premier cas, on parle d'antibiotique bactéricide et dans le second cas d'antibiotique bactériostatique. Lorsque la substance est utilisée de manière externe pour tuer la bactérie par contact, on ne parle pas d'antibiotique mais d'antiseptique. S'il s'agit non pas d'une substance mais d'un virus, on parle de bactériophagique.


Name.

Brussels has two universities whose names mean "Free University of Brussels" in English: the French-speaking Université Libre de Bruxelles and the Dutch-speaking Vrije Universiteit Brussel (VUB). Neither uses the English translation, since it is ambiguous.

History.

When the Belgian State was formed in 1830 by nine breakaway provinces from the Kingdom of the Netherlands, it had three state universities, in Ghent, Liège and Leuven, but no university in the new capital, Brussels. Since the government was reluctant to fund another state university, a group of Freemasons and intellectuals led by Pierre-Théodore Verhaegen and Auguste Baron planned to create a private university, which was permitted under the Belgian Constitution. After the Catholic Church sponsored the foundation of the Catholic University of Mechlin in 1834, the "Université Libre de Belgique" ("Free University of Belgium") opened on 20 November 1834. In 1836, it changed its name to "Université Libre de Bruxelles".  Since 1935, some courses have been taught in both French and Dutch. Beginning in 1963, all faculties offered courses in both languages. In October 1969, shortly after the language dispute at the Catholic University of Leuven, the French and Dutch entities of the ULB separated into two distinct universities. With the act of 28 May 1970, the Vrije Universiteit Brussel and the Université Libre de Bruxelles officially became two separate legal, administrative and scientific entities.  November 20, called "Saint Verhagen" (often shortened to "St V") for Pierre-Théodore Verhaegen, is a holiday for students of both the Université Libre de Bruxelles and the Vrije Universiteit Brussel.

Campuses.

The ULB comprises three main campuses: the "Solbosch" campus, on the territories of Brussels and Ixelles municipalities in the Brussels-Capital Region, the "Plaine" campus in Ixelles, and the "Erasmus" campus in Anderlecht, beside the Erasmus Hospital.  The main and largest campus of the university is the "Solbosch", which hosts the administration and general services of the university. It also includes most of the faculties of the humanities, the "École polytechnique", the large library of social sciences, and among the museums of the ULB, the Museum of Zoology and Anthropology, the "Allende" exhibition room and the M. De Ghelderode Museum.  The "Plaine" campus hosts the Faculty of Science and the Faculty of Pharmacy. There are also the Experimentariums of physics and chemistry, the Museum of Medicinal Plants and Pharmacy and student housing. This site is served by the metro station: Delta.  The "Erasmus" campus houses the Erasmus Hospital and the "Pôle Santé", the Faculty of Medicine, the School of Public Health and the Faculty of Motor Sciences. There is also the School of Nursing (with the Haute école libre de Bruxelles - Ilya Prigogine), the Museum of Medicine and the Museum of Human Anatomy and Embryology. This site is served by the metro station: Erasmus.  The university also has buildings and activities in the Brussels municipality of Auderghem, and outside of Brussels, in Charleroi on the Aéropole Science Park and Nivelles.

International Privileged Partnerships.

University of California, Berkeley, University of Oxford, University of Cambridge, Université de Montréal, Waseda University, Université Pierre et Marie Curie - Paris VI, BeiHang University, Universidade de Sao Paulo, Université de Lausanne, Université de Genève, University Ouaga I Pr. Joseph Ki-Zerbo, University of Lubumbashi

Research.

At the heart of the Free University of Brussels there are at least 2000 PhD students and around 3600 researchers and lecturers who work around different scientific fields and produce cutting edge research.  The projects of these scientists span thematics that concern exact, applied and human sciences and researchers at the heart of the ULB have been awarded numerous international awards and recognitions.  The research carried out at the ULB is financed by different bodies such as the European Research Council, the Walloon Region, the Brussels Capital Region, the National Fund for Scientific Research, or one of the foundations that are dedicated to research at the ULB; the ULB Foundation or the Erasme Funds.

The Free University of Brussels (French; Université Libre de Bruxelles), abbreviated ULB, is a French-speaking private research university in Brussels, Belgium.

L'université libre de Bruxelles (ULB) est une université belge francophone implantée sur trois campus principaux (le Solbosch, la Plaine, Érasme) dans la région de Bruxelles-Capitale, ainsi qu'à Charleroi (Gosselies). Fondée en 1834, elle est l'une des plus importantes universités belges et est régulièrement citée comme faisant partie des 250 meilleures universités mondiales.