190 



B. CHANCE AND L. SMITH 



In order to guard against the possibility that the optical density 

 changes caused by illumination of the solution in Fig. 1 (right) involve 

 different pigments from those of Fig. 1 (left), we have repeated the 

 experiment at various wavelengths and have thereby obtained the 

 difference spectrum plotted in Fig. 2. In this spectrum, the optical 

 density changes recorded on turning the light off are plotted. They 



Reduction due to respirotion Reduction due to hydrosulfite 

 Fig. 1. A comparison of the anaerobic light effect in a suspension of 22. rubrum 

 in which anaerobiosis has been caused by respiration (left) or by hydrosulfite 

 addition (right) . Optical density increases at 430 m/i corresponding to a down- 

 ward deflection of the trace. 



correspond to increases of optical density. This spectrum is essentially 

 the same as that obtained in the absence of hydrosulfite; the same 

 pigments are involved in both cases. 



+0.004 



(in excess SO" ) 



380 



420 



500 



460 



Fig. 2. The difference spectrum caused by infrared illumination of li rubrum 

 suspensions under the conditions of Fig. 1, right. 



