403 Comparative Animal Physiology 



whose velocity varied with the photochemical effect. A measure of the veloc- 

 ity of this reaction could be obtained by measuring the latent period and 

 calculating its reciprocal. Hecht then determined the relation between the 

 reciprocal of the latent period, which he used as a measure of the photo- 

 chemical effect, and the illuminating intensity, using a constant exposure 

 period. The resulting data, when graphed, were adequately described by a 

 straight line with a slope of 2.2. Hecht concluded that the relation could be 



expressed by the formulation 



E = In I 



where E is the photochemical effect, and In I is the natural logarithm of the 

 intensity. This relation is a form of a second fundamental principle in photo- 

 chemistry expressed in Lambert's and in Beer's law. 



REACTIONS FOLLOWING THE INITIAL PHOTOCHEINIICAL EVENT. The end" 



result of the light-induced photochemical reaction is the initiation of activ- 

 ity in the fibers of the optic nerve or, in the absence of a nervous system, 

 the initiation of some response. Hecht was the first to suggest a transitional 

 mechanism between the photochemical event and the response of the organ- 

 ism. This suggestion was based on several characteristics of the latent period 

 of the siphon-withdrawal response. Hecht observed that the reciprocal of the 

 latent period (a measure of velocity) is directly proportional to the duration 

 of exposure, on the basis of which he concluded that the velocity of the la- 

 tent period reaction is directly proportional to the concentration of photo- 

 chemical decomposition products. To Hecht, this linear relation suggested 

 a catalvtic reaction, where the decomposition products were the catalyzers. 

 This assumption was supported by: (1) measurement of temperature coeffic- 

 ients between 2.5 and 3 and a /;i value of 19,680 (calculated from the Ar- 

 rhenius equation), which agrees well with values for catalytic reactions; (2) 

 the consideration that the photochemical decomposition products P -\- A 

 are part of a reversible system whose condition of equilibrium must be sus- 

 tained. This precludes the participation of these products directly in a second 

 reaction. On these evidences and reasons Hecht postulated the following 



scheme 



L II P + A II ^ T 



where L is some substrate changed to an agent T, the reaction being cata- 

 lyzed by the photochemical decomposition products P ~\- A, the parallel 

 vertical lines indicating catalysis. Hecht suggested that T is the agent activat- 

 ing the nervous elements of the eye. 



Participation of Carotenoids in Vision. Light incident upon a photore- 

 ceptor must be absorbed before it can produce a physical or chemical effect. 

 In a review of the researches relating the role of carotenoids to visual func- 

 tion, Wald^"" advanced the thesis that photosensitive processes depend, "al- 

 most universally, upon one distinctive and compact group of substances, the 

 carotenoids." Carotenoids form a distinct group of yellow to red pigments, 

 fat soluble and highly unsaturated, which can be identified by their ab- 

 sorption spectra. Carotenoids, either alone or as the prosthetic groups of con- 

 jugated proteins, function in absorption of radiant energy. 



It is apparent that absorption characteristics of the pigment will be re- 

 flected in the response of the photoreceptor or of the organism. Thus, if the 



