64 L. N. M. DUYSENS 



change in intensity of about 1%, brought about by moving the (cali- 

 brated) wire screen into the beam, caused a deflection of the recorder 

 of 100 to 200 mm. This deflection was compared with the deflection 

 caused by a change in absorption of the suspension upon iUumination 

 with a 500- watt projection lamp, the intensity of which could be 

 varied by varying the lamp voltage. 



The filters /i and /2 are needed to reduce stray light from the pro- 

 jection lamp. In the absence of these filters, the stray light did not 

 bring about a deflection of the recorder, since it was not interrupted 

 with the frequency 60 c.p.s. ; it was found, however, to cause a de- 

 flection when both the measuring and compensating beams were on. 

 The filters were selected to reduce the stray light to such a low level 

 that the projection lamp caused no deflection when a mastic suspen- 

 sion was used, instead of one of photosynthesizing cells. 



The electronic part of the apparatus is shown in Fig. 2. The output 

 of the three-stage amplifier is rectified by the double diode 6AL5, 

 filtered by the four-stage filter, which removes frequencies greater 

 than about one, and, via the balanced cathode follower, passed to a 

 Brown recorder. The multiplier voltage and the filament voltage 

 for the first two stages is obtained from batteries, the 6.3-volt filament 

 voltage for the other stages is obtained from a transformer, and the 

 plate voltage is obtained from a stabilized power supply. 



TYPE OF DATA GIVEN BY MEASUREMENTS OF THE 

 CHANGES IN ABSORPTION 



Figures 3 and 4 give examples of the type of data obtained. The 

 time course of the change in optical density in Chlorella appears to de- 

 pend upon several factors. Figure 3 shows that the initial increase in 

 optical density at 520 mix upon illumination, which is caused by an 

 unknown pigment (cf. 7) is much greater in an anaerobic medium 

 than in an aerobic medium. However, upon illumination for about 1 

 minute, the original increase in the anaerobic medium is followed by a 

 decrease. Figure 4 shows the time course at 420 m/j,, where cytochrome 

 / presumably shows a change in absorption (cf. 8) . The changes are 

 much larger, but less rapid, in the presence of carbon dioxide than in 

 its absence, suggesting that carbon dioxide and cytochrome / compete 

 for the same hydrogen donor. 



A great number of experiments can be made. At each wavelength, 



