752 LIGHT AND LIFE 



I have presented, one might expect that as long as products of the hght 

 reaction that are responsible for \isual excitation remain— perhaps, for 

 example, as long as the site on rhodopsin otherwise covered by 11-m 

 retinene remains exposed— the visual sensation should continue. This should 

 result in a positive after-image; so that the cjuestion involves the decay of 

 the positive after-image. Our ordinary experience of this is that it decays 

 very rapidly; and the first rapid falling-off of the sensation, perhaps only 

 to the point that we can just discriminate from the level of sensation when 

 the light was on, is probably the immediate signal that the light has gone 

 off. For the most part, little attention has been paid to the further decay 

 of the positive after-image. The only real measurements I know are in a 

 paper by Paul Lasareff (Pfiiigers Arch. ges. Physiol., 201, 333, 1923) , in which 

 this decay was measured by Lasareff and Kravkov by two methods. The 

 fairly complete curve of Kravkov's measurements shows the decay of the 

 central (cone vision) after-image to take about 6 minutes; and Lasareff's 

 measurements, showing a rapid decay still in progress after 2 minutes, are 

 consistent with this. Ebbecke's description of the positive after-image, falling 

 rapidly and then more slowly, lasting generally for several minutes, yet not 

 for longer than 6 to 8 minutes after the strongest stimuli— also agrees with 

 these measurements (U. Ebbecke, Pfiiigers Arch. ges. Physiol., 221, 160, 1928- 

 29) . 



According to these measurements and this description, the decay of the 

 positive after-image in human cone vision follows just about the same 

 course as human cone dark adaptation. This suggests that it may indeed 

 be true that after shutting off the light, the visual sensation persists for 

 as long as it takes to regenerate the visual pigment— i.e., as long as opsin 

 remains free or uncovered by W-cis retinene— in the cones. That implies of 

 course that the positive after-image associated with rod vision may last much 

 longer, perhaps for as long as 20 to 30 minutes after shutting off the light. 



Dr. Franc;k: I asked this (juestion for the same reason as Dr. Arnold and 

 Dr. Rabinowitch. The point is, how certain are you that in this case the 

 pigment sits between the layers? 



Dr. Wald: I think what Dr. Franck means to ask by his question is 

 whether it is possible that the retinene is located in a relatively anhydrous 

 situation in the rod structure. I would have to answer yes, that is possible, 

 but do not forget that, for example, we need water to complete fileaching. 

 We need the water to hydrolyze the retinene off of the opsin. 



Dr. Franck: Yes, that is my point. 



Dr. Wald: The next thing that happens is this. As far as we know when 

 we move retinene, it is in the form of vitamin A; so one needs to have it 

 reduced by DPN and alcohol dehydrogenase, which of course demands direct 

 contact. Now, what you just last suggested is to move the excitation, and 

 hence the bleaching and liberation of retinene out to the ends of the layers 

 for this to happen. That is possible, but I can only say that I do not know 

 how to go on from there. 



