referring now more particularly to the drawings , a nephelometer 10 is illustrated for testing bacteria samples to determine whether the samples contain a predetermined bacteria count and hence whether the sample is standardized . accordingly , the apparatus is referred to as a standardized inoculum reader . to determine whether the bacteria samples contain the standardized concentration utilizing the standardized inoculum reader 10 shown in fig1 the reader 10 is calibrated using a sample having a mcfarland standard concentration so that the indicator 11 of the meter 12 has a reading in the central area 14 of the meter face between the &# 34 ;+&# 34 ; area 16 and the &# 34 ;-&# 34 ; area 18 and superposed with line 20 . the mcfarland standard concentration is a solution of barium sulphate ( baso 4 ) particles in a concentration of about 1 × 10 8 particles per ml . after the reader 10 has been calibrated for the mcfarland standard concentration , bacteria samples are tested against the mcfarland standard concentration . bacteria samples for which bacteria counts of from about 0 . 5 × 10 8 to about 5 × 10 8 per ml are desired when tested in the reader 10 will give a meter reading in the area 14 corresponding to an actual bacteria count of from about 1 × 10 8 per ml to about 2 × 10 8 per ml . referring now to fig1 - 3 , the standard inoculum reader 10 includes an optical section 22 which comprises a lamp 24 , an opaque baffle 26 having an orifice 28 therein , another opaque baffle 30 and a photoresistor 32 . the bacteria sample to be analyzed is contained in a sample container 33 in the form of a test tube which is inserted into a receptacle 34 to conduct a test . the optical section 22 is enclosed in an opaque housing 35 to prevent stray light from reaching the sample container . the receptacle 34 includes a pressure sensitive switch 36 ( fig5 ) which is engaged by the bottom of the test tube 33 and which controls illumination of the lamp 24 . depressing the container 33 activates the switch to switch the lamp on , and releasing the container , deactivates the switch to switch the lamp off . the lamp 24 projects light towards the opaque baffle 26 about an optical axis 38 ( fig2 ) which passes through the orifice 28 in a diverging beam 39 . the baffle 30 and the photoresistor 32 are disposed along the optical axis 38 to one side of the container 33 while the lamp 24 and baffle 26 are disposed to the opposite side of the container 33 . the photoresistor 32 is spaced from the receptacle 34 . the space 41 ( fig4 ) between a container 33 in the receptacle and the photoresistor is open except for the disposition of the baffle 30 therein . the diameter of orifice 28 is about two mm while the face of lamp 24 is approximately three cm in diameter . accordingly , beam 39 diverges as it propagates from orifice 28 through container 33 . orifice 28 is beveled to assist in providing a diverging beam . the particles suspended in the mcfarland standard concentration scatter light from beam 39 as it passes through container 33 . for example , light ray 40 of beam 39 is resistor 32 . by virtue of baffle 30 , light within the region bounded by the imaginary lines 43 and passing through the imaginary region shown at 46 , which is transmitted directly ( i . e . without scattering ) from lamp 24 through container 33 toward photoresistor 32 is prevented from intercepting the latter . correspondingly , light from beam 39 scattered within that portion of container 33 opposite baffle 30 also will not reach photoresistor 32 . accordingly , photoresistor 32 only detects light scattered by particles suspended in the sample and falling without that portion of container 33 which is not directly opposite baffle 30 . an upper sleeve 45 , opaque to light in beam 39 , is disposed about the upper portion of container 33 and extends downwardly to a level above the baffle 30 . sleeve 45 serves to minimize errors due to meniscus reflection and scattering , and due to ambient light , and to define the largest detectible scattering angle ( measured with respect to optical axis 38 ) in the upper part of container 33 . a lower sleeve 47 , also opaque to light in beam 39 , is disposed about the lower portion of container 33 and extends upwardly to a level below the baffle 30 . sleeve 47 serves to minimize the effects of bottom focus and scattering , and to define the largest detectible scattering angle in the lower part of container 33 . the mean scattering angle is determined principally by the distance of baffle 30 from container 33 and the distance of baffle 30 from photoresistor 32 , and is preferably selected to be approximately 45 °. by virtue of the characteristic distribution of light intensities over the detectible range of scattering angles , a corona or halo effect illustrated by an imaginary annular region 44 , is observable on the surface of container 33 . baffle 30 which is disposed in space 41 on the optical axis , extends radially thereabout and prevents all light directly transmitted from lamp 24 which passes through the region 46 from reaching the photoresistor 32 . thus , no directly transmitted light is permitted to reach the photoresistor 32 . the baffle 30 and the photoresistor 32 are sized and positioned so that light projected from the annular region 44 is permitted to reach the light sensitive area of the photoresistor 32 , i . e . parts of the sensitive area of the photoresistor are in the field of view of the halo ( fig4 ). nothing is disposed in space 41 except baffle 30 and therefore the light from annular region 44 is directly transmitted through space 41 to the parts of the photoresistor in the field of view 48 of the annular region 44 . the light in the annular region 44 which thus reaches the photoresistor 42 is a measure of the particle count of the barium sulphate or of bacteria in container 33 . the intensity of the light in the annular region 44 is directly proportional to the number of particles or bacteria in the container 33 , i . e . the higher the concentration of particles or bacteria , the more light that is reflected to the annular region 44 . the conductivity of photoresistor 32 is directly proportional to the intensity of the light which impinges upon its light sensitive surface area . with particular reference to fig2 and 3 , a generally rectangular baffle 30 is employed with a cylindrical container , such as container 33 , to eliminate lensing effects of the curved surface of container 33 . that is , light in beam 39 which is refracted at the surface of container 39 but not scattered by particles in the sample will be prevented from reaching photoresistor 32 by baffle 30 . by virtue of the rectangular shape of baffle 30 , light in the upper region 44a and the lower region 44b of region 44 is permitted to reach photoresistor 32 . referring now to fig5 the photoresistor 32 is shown connected in a measuring circuit 60 . the pressure sensitive switch 36 is in an on / off switch for the apparatus and connects and disconnects ac power to the apparatus and to the circuit 60 . lines 62 and 63 connect ac power to the switch . depressing container 33 into receptacle 34 activates the pressure sensitive switch 36 to switch power into the circuit 60 and illuminate lamp 24 which is connected between the switched ac power line 62a and the neutral ac line 63 . photoresistor 32 is disposed to receive the light transmitted by lamp 24 and scattered by the mcfarland standard concentration or the bacteria sample in container 33 into the annular halo region 44 , as described above . circuit 60 in addition to optics section 22 includes a rectifier section 68 , a regulator section 70 , a calibrate section 72 and a &# 34 ; zero &# 34 ; section 74 . the rectifier section 68 is connected between the switched ac line 62a and the neutral ac line 63 and comprises resistor r1 connected in series with a rectifier diode d1 , and a filter capacitor c1 connected in shunt with the series - connected resistor r1 and diode d1 , and the neutral line 63 . rectifier section 68 operates in conventional fashion to provide half - wave rectified dc at the output 76 of the rectifier section . the half - wave rectified dc is fed to the regulator section 70 which comprises a limiting resistor r2 connected at one terminal thereof to diode d1 , and a zener diode zd1 connected in shunt to the other terminal of resistor r2 and neutral line 63 . the regulator section 70 operates in conventional fashion to provide a regulated dc voltage at its output 78 having a value of approximately the zener breakdown voltage of zener diode d1 . the regulated dc voltage is fed to series - connected resistors r3 and r4 which are connected across the output 78 of the regulator section . resistor r4 is a potentiometer whose setting determines the dc voltage at the wiper arm 80 of potentiometer r4 . resistors r3 and r4 are connected as a voltage divider to provide a divided dc voltage at point 82 . resistor r4 is adjusted as will be described below to provide a calibrated output reading on meter 65 . the wiper arm 80 of the potentiometer r4 is connected to one terminal 83 of the photoresistor 32 and the other terminal 84 of the photoresistor 32 is connected to one terminal 85 of meter 65 . the other terminal 86 of meter 65 is connected to the neutral line 63 . the current which flows through meter 65 is determined by the conductivity of photoresistor 32 and the setting of potentiometer r4 , and by the zero section 74 . the zero section comprises potentiometer r6 connected across the switched ac line 62a and the neutral ac line 63 , a diode d2 connected to the wiper arm 87 of potentiometer r6 and to resistor r7 which is connected in series with diode d2 and terminal 85 of meter 65 . resistor r7 and diode d2 provide a negative half - wave rectified voltage which is coupled to meter 65 . the value of the half - wave rectified dc voltage is determined by adjustment of potentiometer r6 . referring now to fig6 another embodiment of an optical section 22a is illustrated . a lamp 90 and a negative lens 92 cooperate to provide a diverging beam of light 39 which is projected towards the container 33 . an opaque baffle 96 is interposed between the lens 92 and the container 33 on the optical axis 38 . the baffle 96 is sized and spaced between the container and lens to permit a portion 39a of the diverging beam of light 39 to reach the container . light 39b propagating towards the container about the axis 38 is blocked by the baffle 96 and prevented from reaching the container 33 . the diverging light 39a which enters the sample is scattered in a manner similar to that described for the embodiment of fig1 - 4 producing the halo region 44 . the light in the halo region 44 is permitted to reach the photoresistor 32 , while the light 39b propagating about the about axis 38 is blocked and accordingly does not reach the photoresistor 32 . as for the embodiment of fig1 - 4 , the intensity of the light in the annular region 44 will be directly proportional to the number of particles or bacteria in the container 33 . the standardized inoculum reader 10 is calibrated at the factory or in the field as follows . a container 33 holding a &# 34 ; zero &# 34 ; sample , i . e . a sample having no particles or a negligible number of particles and which will not produce the halo 44 , is inserted into receptacle 34 and is depressed to activate the system . photoresistor 32 receives no light and its conductivity is therefore at a minimum . potentiometer r6 which is accessible from the exterior of housing 35 ( fig1 ) is adjusted to provide a meter reading which coincides with line 18a at the extreme left of the minus area of the meter face . line 18a indicates a &# 34 ; zero &# 34 ; particle count . the &# 34 ; zero &# 34 ; sample container is removed and a container 33 holding a mcfarland standard concentration sample is inserted in receptacle 34 and depressed to activate the system . potentiometer r4 ( accessible from the exterior of housing 35 ( fig1 )) is adjusted to provide a meter reading coinciding with line 20 in the central region 14 of the meter . the mcfarland standard concentration sample container is removed and the &# 34 ; zero &# 34 ; sample container is reinserted and the potentiometer r6 readjusted to provide the &# 34 ; zero &# 34 ; indication . the &# 34 ; zero &# 34 ; sample container is again removed and the mcfarland standard concentration container is again inserted and the potentiometer r4 again adjusted for a meter reading coinciding with line 20 . since there is some interaction between potentiometer r4 and potentiometer r6 , the last two steps may be repeated until no further adjustment is necessary . standard inoculum reader 10 is now factory calibrated to compare bacteria counts to the particle count of a mcfarland standard concentration . in use , however , it may be necessary to zero and calibrate the system using &# 34 ; zero &# 34 ; and mcfarland standard concentrations and adjusting potentiometers r6 and r4 to insure continued accuracy . after being calibrated as described above , the standard inoculum reader 10 is used to determine whether bacteria samples have a predetermined bacteria count and hence may be used as standardized samples . these are &# 34 ; go &# 34 ;, &# 34 ; no - go &# 34 ; tests . the container 33 containing the bacteria sample to be tested is inserted into the reader 10 and depressed to activate the pressure sensitive switch 36 to activate the system . the intensity of the light in the halo or annular region 44 is directly proportional to the bacteria count of the sample , i . e . the more intense the light in halo region 44 is , the higher the number of bacteria in the sample . photoresistor 32 has a conductivity which is directly proportional to the intensity of light received on its light effective surface area and the more conductive that photoresistor 32 is , the more current that is fed to meter 65 . thus , the more intense the light in the annular region 44 is , the more current is supplied to meter 65 . accordingly , higher bacteria counts will read in the right region 16 of the meter while lower bacteria counts will read in the left region 18 of that meter . thus , if the meter reading is between the &# 34 ;+&# 34 ; and &# 34 ;-&# 34 ; area of the meter , i . e . in the central region 14 , then the bacteria sample contains approximately the predetermined number or count of bacteria and can be used as a standardized concentration . if the meter reads in the &# 34 ;-&# 34 ; area 18 , then the number or count of bacteria is lower than the predetermined number and if the meter reads in the &# 34 ;+&# 34 ; region 16 to the right of the central region , then the bacteria count of the sample exceeds the predetermined number . while the baffles 30 and 96 have been shown spaced from the container 33 , it is contemplated that they may be placed elsewhere in accordance with the optical geometry of a particular system , for example on a container surface . additionally , it is contemplated that optical systems other than the lamp 24 and baffle 26 , and the lamp 90 and lens 92 , may be utilized to provide a diverging beam of light 39 . the advantages of the present invention as well as certain changes and modifications of the disclosed embodiments thereof will be readily apparent to those skilled in the art . it is the applicants &# 39 ; intention to cover by their claims all those changes and modifications which can be made to the embodiment of the invention herein chosen for the purposes of the disclosure without departing from the spirit and scope of the invention .