262 SIR G. STOKES, BART., ON THE PERCEPTION OF COLOUR. 



the molecules of the structure of the nerve itself, but it 

 seems much more probable that it is in something with 

 which that part of the retina is suffused, it may be in the 

 so-called visual purple, which, as experiment shows, is very 

 readily affected by light, changing colour and being ultimately 

 bleached, which indicates a successiA^e formation under the 

 influence of light of different chemical substances. Now it 

 may be that these different chemical substances affect the 

 three classes of nerves differently, and that it is in that way 

 that the differentiation between the stimulation of the 

 different sets is effected at the distal end of the nerve 

 apparatus. If the excitement of the nerves at the distal 

 end is due to substances produced in the photo-chemical 

 action, there must be some difference or other between the 

 three classes of nerves, or between something belonging to 

 them, in order to account for their not being all excited in 

 the same proportion one to another Avhatever be the nature 

 of the exciting light. , It may bo that the nerves are 

 differently constituted in some respects ; but I do not think 

 it is absolutely necessary to suppose even that. I can 

 conceive that it is possible (it is merely an idea that ha& 

 occurred to myself, and I hardly venture to throw it out, 

 especially in the presence of Lord Lister, but if I go wrong- 

 he will correct me in the end) that if the excitement of 

 these nerve fibres is due to chemical stimulation, produced 

 by products of the action of light on the visual purple or 

 some other substance, and if endosmose comes in as well as. 

 photo-chemical action, I can conceive, I say, that without 

 any necessary difference in the structure of the nerves of 

 those three classes, the difference may be made by their 

 position in the outer segments of the rods or cones ; by the 

 nerve fibres lying closer to the surface or a little deeper in.. 

 The minuteness of the rods and cones is such that any 

 substance Avhich is produced at the outside might very 

 quickly pass in by endosmose, and so sensation might very 

 readily respond to the light as the substance is producecl. 

 That, however, is a mere conjecture of my own ; but I will 

 ask Lord Lister to tell the members of the Institute it is 

 all nonsense if he thinks it is. 



I will now refer to one or two very curious recent experi- 

 ments, not directly relating to light, but bearing on other- 

 sensations, and going to establish, or at any rate to confirm, 

 a law, Avhich if it be true seems to have a very important 

 bearing on the theory of our sensations of colour. My 



