722 LIGHT AND LIFE 



Dr. Rushton: There is no way in which I can answer that without bleach- 

 ing as far as I can and then cutting out the eye and extracting it. Or better 

 yet, get Dr. Wald to do it, which would be a much more practical thing. But 

 that hasn't been done. Can you answer this, Dr. Wald? 



Dr. Wald: I think it does almost entirely bleach. I think that when Dr. 

 Rushton says that he has bleached it all or has bleached as much as he can, 

 he probably bleached almost all of it. In any case, it can almost all be 

 bleached, as Dowling has recently shown by the direct extraction and estima- 

 tion of rhodopsin from the retinas of light-adapted rats (cf. Wald's paper in 

 this volume) . 



Dr. Franck: I want to say why I asked this question. Could the structure 

 of the rods and the cones be such that every part of the cones is in one 

 way or another directly connected to the nerve, but in the rods the light is 

 absorbed by the dye in the region where there is no nerve and then trans- 

 mitted through the dye to the region of the nerve? In that case this would 

 be similar to the photosynthetic unit in photosynthesis. In that case it 

 would seem that only a small part of the dye would l)e involved in the 

 photochemistry. 



Dr. Rushton: I do not think that I have enough understanding of the 

 molecular arrangement to make any comment which would be worth while. 



Dr. MacNichol: There is something which I do not understand. Is the 

 reason that you do not get this nice simple relation between rod bleaching 

 and sensitivity in the normal case due to some kind of neural interaction 

 between the rods and cones? 



Dr. Rushton: No, I think not. Dr. MacNichol asks, if I understand 

 aright, why in the normal subject we cannot get evidence of rod activity 

 early in the course of dark adaptation, and have to go to a rod monochromat 

 to investigate it. It has often been suggested that cones inhibit rods, and 

 if this ivere the reason that rods are not normally apparent in the light- 

 adapted state it is easy to see that a rod monochromat who lacks cones 

 would also lack rod inhibition, and hence rod activity would be manifest. 

 I believe that this explanation is false and that the true explanation is 

 simpler, for if (a) there is no rod-cone interaction we may explain the 

 results satisfactorily whereas if (b) cones inhibit rods we are led to expecta- 

 tions which are not fulfilled. 



(a) Dark adaptation is a threshold measurement and hence will simply 

 record the more excitable of two independent visual mechanisms. This 

 at first is the cones, which do not saturate at high intensities and which recover 

 much faster. The threshold of the rods, therefore, cannot he measured until 

 it falls below that of the cones, which does not happen until about 18 minutes 

 in the dark after a total bleach. In the rod monochromat, however, there are 

 no cones to steal the threshold, and we obtain the full, unobstructed rod 

 recovery curve, which as Fig. 5 shows, coincides with the normal rod curve 

 when that i)ecomcs visil)lc after 18 minutes. 



