VISUAL RECEPTORS AS BIOLOGICAL TRANSDUCERS 61 



concentration. I want very much to do this and to try to study these very re- 

 lationships that you have posed. 



Dr. Johx Hughes (National Institutes of Health): Regarding the relation- 

 ship between the slow potential as recorded from ommatidia and the spike 

 discharge from the fiber, I was wondering if you have been able to demonstrate 

 an I.xT. constant as was demonstrated by Dr. Hartline's experiments some 



years ago? 



Dr. MacNichol: ^Ye haven't studied this specifically, but I am quite sure 



that it exists. 



Dr. Dan Bradley (National Institute of Mental Health): If I understood 

 your talk, potassium ions seemed to be implicated in dark adaptation; is that 

 right? 



Dr. MacNichol: Yes. 



Dr. Bradley: Well, if this is so, we would expect that, if the light intensity 

 in the light interval preceding the dark adaptation period were reduced suffi- 

 ciently, the diffusion reaction would proceed faster than the light reaction, 

 which would obviate the necessity for dark adaptation. Has such a threshold 

 been observed? 



Dr. MacNichol: I haven't done any experiments, but I do not think that 

 would bear on that. In our experiments I showed that we light adapted with 

 as much light as we had available for ten minutes. But I have not done any 

 work over longer periods at lower intensity. 



Dr. Gilbert Ling (University of Illinois): I would like to make some com- 

 ments. My ideas spring from our work on muscles. I wonder if Dr. Dethier 

 would discuss with us whether such ideas are applicable. In this particular 

 experimental material the idea was put forth several years ago that the selec- 

 tive ionic accumulation in the cells, as well as ionic permeability of the cell 

 surface, could be explained by the hypotheses of a fixed charge system and the 

 difference in interaction energy between such fixed charges and the definitely 

 hydrated ions. The thesis was rather lengthy, but, in general, you may say 

 that the sequence of ionic effectiveness or the sequence of ionic permeability 

 and accumulation agree exactly with the sequence that you have seen as far 

 as the cation threshold is concerned. Therefore, it turns, firstly, to why are 

 these different ions— why do they have different effects in the strength of 

 stimulation? The second is, why do they cause the excitation? 



It is possible to think that because of the difference in the energy between 

 the fixed charges, which themselves are fixed, and the differently hydrated 

 ions, which vary in polar energy according to their size, therefore a larger 

 hydrated ion like sodium would have less effect than that of a smaller one, 

 namely, potassium. 



Now, the next question is, how would they initiate the electrical response? 



