276 W- J- Crozier, 



changed by light. When an animal has been for some time in 

 sunlight which does not exceed a certain "optimum" intensity^ 

 acclimitization is brought about by the fact that the rate of photo- 

 lysis of this substance and the rate of its reconstruction by the 

 activity of the tissue cells are so related that the actual concentra- 

 tion of the photochemical receptor does not change appreciably, or 

 else changes at a very slow rate. It is further assumed that either 

 the photolytic products of this substance, or the changing concen- 

 trations of the photosensitive material itself, constitute a stimulus 

 for nerve net processes associated with the pigment cells. If, now, 

 the rate of photolysis be changed by increasing the intensity of 

 the light, the acclimitization balance will be disturbed, and a sti- 

 mulus thereby produced. On the other hand, if a rapid decrease 

 in the light intensity is caused, by casting a shadow, the process 

 of photolysis will abruptly cease, or fall to a very low rate, and 

 the unopposed reconstruction process will initiate the shadow reflex. 

 The concentration of the photo-sensitive material present at any 

 given temperature and time may, for the moment, be considered as 

 the result of a reversible reaction sj^stem 



P^a + b + ... 

 in which the rate of recombination of a + b -|- ... in the absence 

 of light is much greater than the rate of photolysis of P. The 

 more rapid recombination of a -f b + . . . in the absence of light, 

 which is assumed, may be accounted for in several ways. Light 

 may in part hinder the formation of the photosensitive material by 

 tending to side-track some of its constituents (a, b, etc.) in other 

 photochemical processes. Inasmuch as colloidal systems are con- 

 cerned, it is almost certain that false equilibria (surface effects) are 

 also to be reckoned with. A gross parallel of the state of affairs 

 here pictured may be found in the migration of pigment granules 

 in the retinular cells of Crustacea and melanophores of vertebrates 

 (cf. Paekee, 1906), where the migration of the melanin is more 

 rapid in the distal than in the proximal direction. A closer parallel 

 is the behavior of the rod-pigment (visual purple) of vertebrates. 

 It will likewise at once be recognized that this idea of a complex 

 photochemical equilibrium, when more precisely developed, may be 

 of considerable significance in the discussion of the HEEiisra theory 

 of color vision, the photolysis and reconstruction phases of the re- 

 action corresponding to what psychologists refer to in the terms 

 katabolism and anabolism. 



