494 



Dr. G. J. Burch. On Light- 



[Apr. 19, 



we must carefully separate the initial electro-chemical action from the 

 resultant physiological action. The production of the active decomposition 

 products which excite the retina takes place, doubtless, according to formulse 

 capable of exact and more or less simple numerical expression. The 

 regeneration of the sensitive material, whether due to a natural tendency to 

 revert to its original condition or whether effected, as seems more probable, 

 by processes of metabolism, is still conceivably capable of numerical expres- 

 sion. But between these actions, which may be classed as electro-chemical, 

 and the resultant sensation there is a whole group of modifying causes, which 

 must, for the present, be kept outside our attempts at analysis. 



There arises, in this connection, a question of general interest in physiology 

 that has not, so far as I know, been fully dealt with. Is the excitation of a 

 tissue by a chemical compound a process of the same character as a chemical 

 reaction ? I do not refer to such crude foreign substances as dehydrate or 

 coagulate, or otherwise damage the tissue, but such as excite quite normally 

 its characteristic functions. There must be some end to the activity of the 

 exciting substances in the eye, else, once separated, they would go on exciting 

 sensation indefinitely. Either they recombine or are washed away or their 

 energy is transformed into that of the sensation. I do not attempt to 

 decide this problem, but have so stated the theory that it would not be 

 affected by it. 



I. Opto-chemical Processes. 



Let n — the number of molecules of the visual substance not yet acted on 

 by light. 



x = the number of molecules of the decomposition products capable 



of exciting the retina. 

 e = the number of molecules of decomposition products used in pro- 

 ducing; sensation. 



The known data are insufficient for a complete statement of the problem — 

 for instance, it would be necessary to know whether any molecules of x 

 remain ultimately unaccounted for by e — whether they are neutralised, 

 destroyed, or simply washed away by the circulation and dispersed. 



For our present purpose it is sufficient to note that each of the variables is 

 subject to conditions of equilibrium governed by the ordinary laws. 



Then dnjdt = the rate of supply of the visual substance, 

 and dx/dt = the rate of demand upon it. 



Also de /dt = the physical intensity of the excitation. 



If dnjdt is greater than dx/dt the store n of molecules capable of being 



