420 LIGHT AND LIFE 



where chlorophyll absorbs quite strongly you are already above saturation. 

 This would occur in the upper layers of the suspension that are exposed to 

 the light whereas when you go out further toward the red, the light is 

 transmitted down deeper into the suspensions, giving a higher rate. 



Dr. Kok: Your worries are our worries. To help light penetration we 

 stir as fast as we can. We checked this point again just before the meeting. 

 This work is done with interference filters— to obtain measurable rates we 

 have to use this rather impure monochromatic light. Now, we placed a 

 Schott RG8 glass filter in front of the 700 or 710 interference disc so that 

 all radiation below 690 m/t is cut off. This cuts the total transmitted light 

 by a factor of two or more. Still the rate was higher than that with the full 

 transmission of a 670 interference filter. 



Note added in proof: Perhaps the most convincing evidence [against Dr. 



Strehler's hypothesis] is that the phosphor)'lation associated with ferrocyanide 



reduction and that mediated by riboflavin 5-phosphate or vitamin Ka 



(measured in the same set up under identical conditions to PAIS) does not 



show the maximum at 700-710 m/j.. 



Dr. Strehler: I would like to suggest that you could absolutely rule out this 

 possibility if you were to measure the fluorescence yield under your \arious 

 conditions of illumination. If the fluorescence yields changed in the opposite 

 direction from your transmission measurements or if they didn't change at 

 all, then you would certainly rule them out. 



Dr. Kok: No, that fluorescent measurement wouldn't work. This problem 

 doesn't bother us, because whatever fluorescence is caused by the activating 

 beam the photocell could only see a change in fluorescence induced by the 

 detecting beam and occurring after the flash is over. That's all that is plausi- 

 ble. You do not see fluorescence. Right? That's what you are worried 

 about. Fluorescence after the flash? 



Dr. Strehler: You pick up fluorescence from the measuring beam which 

 should be more intense at the far end of the chlorophyll spectrum. 



Dr. Kok: No, we do not see activating beam fluorescence. We have a 

 phosphoroscope arrangement. Only if the actinic beam should produce an 

 after-effect in the fluorescence of the detecting beam would we see it. But 

 then this fluorescence would be seen by the photocell regardless of the wave- 

 length setting of the monochromatic beam (cf. alcove) . 



Dr. Rabinowitch: May I come back to our old observations on the differ- 

 ence spectrum in Chlorella. Although these results differ somewhat from 

 the ones described by Dr. Kok. our experiments made were under different 

 conditions— in constant light— and therefore do not invalidate each other. 

 I want to answer Dr. Strehler's skeptical question: why other people did not 

 find any effects in the red part of the spcttrum. If one measures the differ- 

 ence spectrum of Chlorella in light of different intensities, one finds that 

 even weak light produces a positive effect at 520 m/*, as well as a negative 

 effect at 480 mfx, (and also the cytochrome effects) . On the other hand, in 



