60 L. N. M. DUYSENS 



optical density ])rought about by illumination. One might try to 

 measure the intensity of the transmitted beam by means of a photo- 

 tube (or multiplier), amplifier, and meter. However, in many experi- 

 ments the changes in intensity must be measured with a precision 

 of the order of 0.01%. In order to do this, a meter of great precision 

 is required; and the intensity of the measuring beam, the sensitivity of 

 the phototube (or multiplier), and the amplifier must be constant 

 within 0.01% — requirements which are difficult to fulfill. 



Apparatus with compensating beam. The required precision can be 

 achieved by using a compensating light beam from the same light 

 source as the measuring beam. The compensating beam should not 

 change its intensity owing to changes in absorption of the suspen- 

 sions but should deflect the meter or recorder opposite to that of the 

 measuring beam. If the measuring and compensating beam are of 

 about equal intensity, changes in the electronic parts of the apparatus 

 will not cause marked deflections of the meter; appreciable deflections 

 will occur only if the transmission of the suspension changes. The 

 sensitivity of this device increases with the intensity of the measuring 

 (or compensating) beam. 



In the instruments (1,2) used so far successfully, the measuring 

 and compensating beams were alternated with line-frequency by 

 means of mechanical devices, such as moving mirrors, and both 

 beams impinged upon the same multiplier or phototube. The output 

 was passed through an a-c amplifier and caused a deflection of a 

 phase- and line-frequency-sensitive device. 



In one type of apparatus (1), the two beams were obtained by 

 splitting the beam leaving the monochromator; the compensating 

 beam bypassed the suspension. A description of such an apparatus 

 will be given below. 



In the second type of apparatus (2), the compensating beam is ob- 

 tained from a second monochromator, and is also passed through the 

 suspension. The wavelength of the compensating beam is fixed and 

 is selected so that no change in absorption upon illumination occurs. 

 The advantages of the latter arrangement are that it compensates to 

 a certain extent for changes in transmission due to sedimentation of 

 the cells, and that it is relatively easy to catch a larger angle of light 

 scattered by the cells. One disadvantage of the second type of appara- 

 tus is that it is affected by changes in lamp output, when the wave- 

 lengths of measuring and compensating beam are different, since the 



