April 15, 1921] 



SCIENCE 



351 



ary state which results when the opposing 

 light and " dark " reactions become balanced, 

 and no fresh decomposition products can be 

 formed by the light. Dark adaptation very 

 obviously is a clear function of the unopposed 

 " dark " reaction. 



More than this, however. Certain predic- 

 tions may be made on the basis of this re- 

 versible reaction. Several of these have been 

 investigated with complete success. To men- 

 tion just a simple example: the "dark" re- 

 action, P -\- A -^ S, is an ordinary chemical 

 reaction; its temperature coefficient should 

 therefore lie between 2 and 3. This is equiv- 

 alent to saying that the temperature coeffi- 

 cient of dark adaptation should lie between 

 2 and 3 for 10° C. This is precisely what has 

 been found to be true. The temperature coeffi- 

 cient of dark adaptation for Mya is 2.4. This 

 concept of a reversible photochemical reaction 

 has therefore been fruitful in accounting for 

 the known properties of photosensory stimu- 

 lation, and has served to suggest the investi- 

 gation of other properties. The results of 

 these have in turn corroborated the original 

 explanation. 



vm 



So far we have considered the events which 

 take place during the sensitization period 

 only. The photosensory responses of these 

 animals, however, involve the very definite 

 existence of a latent period. In fact, in the 

 case of Mya, most of the reaction time is 

 merely latent period and nothing more. For- 

 tunately this part of the reaction time has 

 also yielded to quantitative methods of 

 analysis, and as a result we can now ofier 

 an explanation of photoreception which covers 

 not only the sensitization period, but the 

 latent period as well. 



At the beginning of this paper, in defining 

 the difierent parts of the reaction time, I 

 pointed out a significant fact. It is that if 

 the exposure of an animal to light is made 

 shorter than the sensitization period at that 

 intensity, the reaction time — and consequently 

 the latent period — is prolonged. This indi- 

 cates that there is some interrelation between 

 the two portions of the reaction time. Ex- 



periments were therefore made in which ani- 

 mals were exposed for varying periods of 

 time, all less than the sensitization period. It 

 was found that the duration of the latent 

 period varies inversely with the length of the 

 exposure to light. 



The latent period, being the interval during 

 which the animal may remain in the dark 

 following the exposure, is certainly not a 

 time during which nothing happens. We 

 may be sure that a process takes place during 

 the latent period which is in some way a vital 

 link in the chain of events between the inci- 

 dence of the light and the appearance of the 

 response. Whatever this process may be, we 

 can consider its velocity as proportional to 

 the reciprocal of the duration of the latent 

 period. When this is done, we find that the 

 velocity of the latent period process is a 

 linear function of the duration of the initial 

 exposure to light. 



During the exposure we know that the 

 photosensitive substance 8 is decomposed. 

 We may assume that for these extremely 

 small exposures, the photochemical effect is 

 directly proportional to the time of action of 

 the light. It therefore follows that the 

 velocity of the latent period process is a linear 

 function of the photochemical effect during 

 the exposure. In other words, the velocity of 

 the latent period reaction is directly propor- 

 tional to the concentration of freshly formed 

 precursor substances P and A. 



Such a relationship may be explained in 

 several ways. The one finally chosen assumes 

 that during the latent period an inert sub- 

 stance, L, is changed into a chemically active 

 material, T, which then acts upon the nerve 

 to produce the outgoing sensory impulse. 

 This reaction, L—>T, is catalyzed by the 

 presence of the freshly formed photochemical 

 decomposition products, P and A, formed 

 during the exposure to the light. The linear 

 relation between velocity of reaction and con- 

 centration of catalyst is a very co mm on one 

 in catalyzed reactions. 



In terms of this conception the latent 

 period assumes a position of prime importance 

 in the photosensory mechanism. The latent 



