106 1'. LATIMER AND E. I. KABIKOWITCH 



effect of scattering is negligible, since the pigment concentration is not sufficient 

 to give steep gradients of the refractive index. We think therefore that our meas- 

 urements were not influenced by scattering. As a further control, one can add a 

 pigment of known optical properties to the scattering suspension and see if the 

 scattering distorts its spectrum. (We used cytochrome for this purpose.) One can 

 then add a detergent which will clarify the suspension; with whole cells, one may 

 also add lysogen, if they are susceptible to its action. We have done this in every 

 case, and compared the spectra before adding the chelating agent and after, 

 and the spectra before and after adding lysogen. Chemical tests also are of assist- 

 ance: in Rhodospinlluni, where phototaxis occurs, we would be worried, if it 

 were not for the fact that we did get similar effects with oxygen as with light, 

 and that the spectral changes observed corresponded to those in isolated cyto- 

 chromes. 



I completely agree with Latimer's remarks when it comes to small transmission 

 changes in highly pigmented particles, e.g., Chlorella. 



Benson: I think that the technique Dr. Shibata has developed, using opal 

 glass, may be useful in testing for this effect. 



Latimer: Instead of using opal glass, which collects most of the scattered light, 

 one could also use an integrating sphere, which collects all of it. 



Frenkel: Dr. Shibata, did your results with opal glass and with the sphere 

 agree? 



Shibata : I believe that you can get better results using the opal glass, although 

 the differences are small. The opal glass method requires no special instruments. 



James Smith : In very fine suspensions of protochlorophyll in its natural state 

 we got no improvement at all with opal glass. Do you think that this was due to 

 the smallness of the particles? 



Shibata : I think so. 



Duysens: I should like to remark that Latimer measured light perpendicular 

 to the incident beam. For most dilute suspensions, 95% of scattering is in the 

 direction of the beam. 



Latimer: I find the same effect at angles from 45 to 135 degrees. 



Duysens: Most of the scattering occurs within 35 degrees (or less) to the inci- 

 dent beam. 



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