IBN SINA (AVICENNA) (980-1037)

‘al Qann fi al-Tibb’ (The Canon of Medicine), also ‘ The Book of the Remedy

Avicenna lived under the Sammarid caliphs in Bukhara. He identified different forms of energy – heat, light and mechanical – and the idea of a force.

drawing of Ibn Sina ©


Before GALEN, scientists describing nature followed the old Greek traditions of giving the definitions and following them up with the body of logical development. The investigator was then obliged merely to define the various types of ‘nature’ to be found. With Galen this procedure was changed.

Instead of hunting for these natures and defining more and more of them, reproducing ARISTOTLE’s ideas, AVICENNA, a Persian physician, planned inductive and deductive experimental approaches to determine the conditions producing observable results.

His tome surveyed the entire field of medical knowledge from ancient times up to the most up to date Muslim techniques. Avicenna was the first to note that tuberculosis is contagious; that diseases can spread through soil and water and that a person’s emotions can affect their state of physical health. He was the first to describe meningitis and realize that nerves transmit pain. The book also contained a description of 760 drugs. Its comprehensive and systematic approach meant that once it was translated into Latin in the twelfth century it became the standard medical textbook in Europe for the next 600 years.

Arabic Canon of Medicine by Avicenna 1632. Many physicians in the Islamic world were outstanding medical teachers and practitioners. Avicenna (980-1037 CE) was born near Bokhara in Central Asia. Known as the 'Prince of Physicians', his Canon of Medicine (medical encyclopedia) remained the standard text in both the East and West until the 16th century and still forms the basis of Unani theory and practice today. Divided into five books, this opening shows the start of the third book depicting diseases of the brain.

Arabic Canon of Medicine by Avicenna 1632

Avicenna thought of light as being made up of a stream of particles, produced in the Sun and in flames on Earth, which travel in straight lines and bounce off objects that they strike.

A pinhole in a curtain in a darkened room causes an inverted image to be projected, upside-down, onto a wall opposite the curtained window. The key point is that light travels in straight lines. A straight line from the top of a tree some distance away, in a garden that the window of the camera obscura faces onto – passing through the hole in the curtain – will carry on down to a point near the ground on the wall opposite. A straight line from the base of the tree will go upwards through the hole to strike the wall opposite near the ceiling. Straight lines from every other point on the tree will go through the hole to strike the wall in correspondingly determined spots, the result is an upside-down image of the tree (and of everything else in the garden).

He realized that refraction is a result of light traveling at different speeds in water and in air.

He used several logical arguments to support his contention that sight is not a result of some inner light reaching outward from the eye to probe the world around it, but is solely a result of light entering the eye from the world outside – realizing that ‘after-images’ caused by a bright light will persist when the eyes are closed and reasoning that this can only be the result of something from outside affecting the eyes. By effectively reversing the extro-missive theory of Euclid, he formulated the idea of a cone emanating from outside the eye entering and thus forming an image inside the eye – he thus introduced the modern idea of the ray of light.

The idea which was to have the most profound effect on the scientific development of an understanding of the behaviour of light was the thought of the way images are formed on a sunny day by the ‘camera obscura’.

AL HAZEN (c.965-1039)

Born in Basra and working in Egypt under al-Hakim, Abu Ali al-Hassan ibn al-Haytham was one of the three greatest scientists of Islam (along with al-Biruni and ibn-Sina). He explained how vision works in terms of geometric optics and had a huge influence on Western science. He is regarded as one of the earliest advocates of the scientific method.

The mathematical technique of ‘casting out of nines’, used to verify squares and cubes, is attributed to al-Hazen.

Al-Hazen dissented with the J’bir Ayam hypothesis of transmutation, thus providing two different strands for Alchemy in Europe from the Islāmic world.

diagram explaining the working of the eye

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1502 – Florence, Italy

‘In the Renaissance science was reinvented’

Image of the VITRUVIAN MAN


Leonardo is celebrated as the Renaissance artist who created the masterpieces ‘The Last Supper’ (1495-97) and ‘The Mona Lisa’ (1503-06). Much of his time was spent in scientific enquiry, although most of his work remained unpublished and largely forgotten centuries after his death. The genius of his designs so far outstripped contemporary technology that they were rendered literally inconceivable.

The range of his studies included astronomy, geography, palaeontology, geology, botany, zoölogy, hydrodynamics, optics, aerodynamics and anatomy. In the latter field he undertook a number of human dissections, largely on stolen corpses, to make detailed sketches of the body. He also dissected bears, cows, frogs, monkeys and birds to compare their anatomy with that of humans.

It is perhaps in his study of muscles where Leonardo’s blend of artistry and scientific analysis is best seen. In order to display the layers of the body, he developed the drawing technique of cross-sections and illustrated three-dimensional arrays of muscles and organs from different perspectives.

Leonardo’s superlative skill in illustration and his obsession with accuracy made his anatomical drawings the finest the world had ever seen. One of Leonardo’s special interests was the eye and he was fascinated by how the eye and brain worked together. He was probably the first anatomist to see how the optic nerve leaves the back of the eye and connects to the brain. He was probably the first, too, to realise how nerves link the brain to muscles. There had been no such idea in GALEN’s anatomy.

Possibly the most important contribution Leonardo made to science was the method of his enquiry, introducing a rational, systematic approach to the study of nature after a thousand years of superstition. He would begin by setting himself straightforward scientific queries such as ‘how does a bird fly?’ He would observe his subject in its natural environment, make notes on its behaviour, then repeat the observation over and over to ensure accuracy, before making sketches and ultimately drawing conclusions. In many instances he would directly apply the results of his enquiries into nature to designs for inventions for human use.

Self portrait of LEONARDO DA VINCI


He wrote ‘Things of the mind left untested by the senses are useless’. This methodical approach to science marks a significant stepping-stone from the DARK AGES to the modern era.

1469 Leonardo apprenticed to the studio of Andrea Verrocchio in Florence

1482 -1499 Leonardo’s work for Ludovico Siorza, the Duke of Milan, included designs for weaponry such as catapults and missiles.
Pictor et iggeniarius ducalis ( painter and engineer of the Duke )’.
Work on architecture, military and hydraulic engineering, flying machines and anatomy.

1502 Returns to Florence to work for Pope Alexander VI’s son, Cesare Borgia, as his military engineer and architect.

1503 Begins to paint the ‘Mona Lisa’.

1505-07 Wrote about the flight of birds and filled his notebooks with ideas for flying machines, including a helicopter and a parachute. In drawing machines he was keen to show how individual components worked.

1508 Studies anatomy in Milan.

1509 Draws maps and geological surveys of Lombardy and Lake Isea.

1516 Journeys to France on invitation of Francis I.

1519 April 23 – Dies in Clos-Luce, near Amboise, France.

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1621 – Holland

woodblock print portrait of WILLEBRORD SNELL ©


‘During refraction of light, the ratio of the sines of the angles of incidence ( i ) and refraction ( r ) is a constant equal to the refractive index of the medium’

In equation form: n1sini = n2 sinr 
where n1 and n2 are the respective refractive indices of the two media.

The refractive index of a substance is a measure of its ability to bend light. The higher the number the better light is refracted. The refractive index of diamond, 2.42, is the highest of all gems.

Refraction is the change in direction of a ray of light when it crosses the boundary between two media. It happens because light has different speeds in different media. A ray of light entering a medium where the speed of light is less (from air to water, for example) bends towards the perpendicular to the boundary of the two media. It bends away from the perpendicular when it crosses from water to air. Refraction was known to ancient Greeks, but Snell, a Dutch mathematician, was the first to study it.

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RENE DESCARTES (1596-1650)

1637 – France

Cogito ergo sum‘ – The result of a thought experiment resolving to cast doubt on any and all of his beliefs, in order to discover which he was logically justified in holding.

Descartes argued that although all his experience could be the product of deception by an evil daemon, the demon could not deceive him if he did not exist.

His theory that all knowledge could be gathered in a single, complete science and his pursuit of a system of thought by which this could be achieved left him to speculate on the source and the truth of all existing knowledge. He rejected much of what was commonly accepted and only recognised facts that could intuitively be taken as being beyond any doubt.

His work ‘Meditations on First Philosophy’ (1641) is centered on his famous maxim. From this he would pursue all ‘certainties’ via a method of systematic, detailed mental analysis. This ultimately led to a detached, mechanistic interpretation of the natural world, reinforced in his metaphysical text ‘Principia Philosophiae‘ (1644) in which he attempted to explain the universe according to the single system of logical, mechanical laws he had earlier envisaged and which, although largely inaccurate, would have an important influence even after Newton. He envisaged the human body as subject to the same mechanical laws as all matter; distinguished only by the mind, which operated as a distinct, separate entity.

Through his belief in the logical certainty of mathematics and his reasoning that the subject could be applied to give a superior interpretation of the universe came his 1637 appendix to the ‘Discourse’, entitled ‘La Geometrie‘, Descartes sought to describe the application of mathematics to the plotting of a single point in space.

This led to the invention of ‘Cartesian Coordinates’ and allowed geometric expressions such as curves to be written for the first time as algebraic equations. He brought the symbolism of analytical geometry to his equations, thus going beyond what could be drawn. This bringing together of geometry and algebra was a significant breakthrough and could in theory predict the future course of any object in space given enough initial knowledge of its physical properties and movement.

Descartes showed that circular motion is in fact accelerated motion, and requires a cause, as opposed to uniform rectilinear motion in a straight line that has the property of inertia – and if there is to be any change in this motion a cause must be invoked.

By the 1660s, there were two rival theories about light. One, espoused by the French physicist Pierre Gassendi (1592-1655) held that it was a stream of tiny particles, traveling at unimaginably high-speed. The other, put forward by Descartes, suggested that instead of anything physically moving from one place to another the universe was filled with some material (dubbed ‘plenum’), which pressed against the eyes. This pressure, or ‘tendency of motion’, was supposed to produce the phenomenon of sight. Some action of a bright object, like the Sun, was supposed to push outwards. This push was transmitted instantaneously, and would be felt by the human eye looking at a bright object.

There were problems with these ideas. If light is a stream of tiny particles, what happens when two people stand face-to-face looking each other in the eye? And if sight is caused by the pressure of the plenum on the eye, then a person running at night should be able to see, because the runner’s motion would make the plenum press against their eyes.

Descartes original theory is only a small step to a theory involving pulses of pressure spreading out from a bright object, like the pulses of pressure that would travel through water if you slap the surface, and exactly equivalent to pressure waves which explain how sound travels outward from its source.

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ISAAC NEWTON (1642-1727)

1687 – England


portrait of NEWTON ©


The force is known as gravitation
Expressed as an equation:

F = GmM/r2




Newton’s ideas on universal gravitation did not emerge until he began a controversial correspondence with ROBERT HOOKE in around 1680. Hooke claimed that he had solved the problem of planetary motion with an inverse square law that governed the way that planets moved. Hooke was right about the inverse square law, but he had no idea how it worked or how to prove it, he lacked the genius that permitted Newton to combine Kepler’s laws of planetary motion with the assumption that an object falling towards Earth was the same kind of motion as the Earth’s falling toward the Sun.

It amounts to deriving Kepler’s first law by starting with the inverse square hypothesis of gravitation. Here the Sun attracts each of the planets with a force that is inversely proportional to the square of the distance of the planet from the Sun. From Kepler’s second law, the force acting on the planets is centripetal. Newton says this is the same as gravitation.


Title page of Philosophiae Naturalis Principia Mathematica

Title page of Philosophiae Naturalis Principia Mathematica



Why do two objects attract each other?
‘I frame no hypotheses’, said Newton




1687 – England




The first law

Weight is not the same as mass.

The second law


The third law









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1860 – Germany

‘Each chemical element, when heated to incandescence, produces its own characteristic lines in the spectrum of light’

For example, sodium produces two bright yellow lines.
Bunsen developed the Bunsen burner in 1855.
In the flame test the Bunsen burner’s non-luminous flame does not interfere with the coloured flame given off by the sample.


Kirchhoff was a professor of physics at Heidelberg. Bunsen and Kirchhoff together developed the first spectroscope, a device used to produce and observe a spectrum. They used their spectroscope to discover two new elements, caesium (1860) and rubidium (1861).

In 1860 Kirchhoff made the discovery that when heated to incandescence, each element produces its own characteristic lines in the spectrum.

This means that each element emits light of a certain wavelength – sodium’s spectrum has two yellow lines (wavelengths about 588 and 589 nanometres). The Sun’s spectrum contains a number of dark lines, some of which correspond to these wavelengths.

The Swedish scientist ANDERS ANGSTROM had, four years earlier, found that a gas always absorbs light at the same wavelength that it emits light. If the gas is hotter than the light source, then more light is emitted by the gas than absorbed, creating a bright line in the spectrum of the light source. If the gas is cooler than the light source the opposite happens; more light is absorbed by the gas than is emitted, creating a dark line.
The dark solar D lines told Kirchhoff that sodium is present in the relatively cool outer atmosphere of the Sun. This could be tested in the laboratory by burning a piece of chalk in a hot oxygen-hydrogen torch. The intensely bright limelight that is produced may be passed through a cooler sodium flame and the light emerging examined through a spectroscope. Crossing the spectrum of the artificial light occur black lines at the same wavelength that a sodium flame emits light. This solved the mystery of the FRAUNHOFER LINES.

Scientists now had a means to determine the presence of elements in stars. By comparing the dark lines in the spectra of light from the stars with the bright lines produced by substances in the laboratory, Kirchhoff had been able to identify the elements that made up a celestial body millions of miles away in space.



In England the astronomer William Huggins recorded the spectra of hundreds of stars and showed the unmistakable fingerprints of familiar elements that are found on the Earth’s surface. The stars are made of exactly the same kind of atoms as the Earth.

In 1868 Norman Lockyer described a spectral line in the yellow region very close to the wavelength of the two ‘D’ spectral lines of sodium. After repeated attempts to discover a substance that produced the same line on Earth, it appeared that the line did not correspond to any hitherto known element. Lockyer gave the element the name ‘helium’, the gas later to be found associated with radioactive decay in ores containing uranium.


Helium had not previously been found on Earth because it is both inert and lighter than air, ironic because after hydrogen, helium is the second most common element in the universe.

In 1904 RUTHERFORD would declare that the presence of helium in the Sun was evidence that sunlight was a product of radioactive processes. The absence of any FRAUNHOFER lines in sunlight that corresponded to radium dealt a blow to this hypothesis. Was there another way of releasing atomic energy than radioactivity?


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