the Sense of Sight. 1 73 



laterally among the crowds higher up the gangways between 

 the rods, when the pressure would at once be increased in the 

 region of these smaller corpuscles by infinitesimal movements 

 of the pigment-granules from both directions. Tn this way 

 it would be possible to have rods dispersing laterally each 

 colour of the solar spectrum in succession, beginning from the 

 top with the violet. How many different zones there actually 

 are in the longest tapering rods ('' cones") in the human eye 

 can only perhaps be ascertained by a careful analysis of our 

 sensations. It was long ago shown (Young-Helmholtz theory) 

 that three elementary sensations — red, green, and violet — would 

 be sufficient to explain tiie rest ; but, according to our theory, 

 there appears to be no reason why there might not be more. 



According to this theory white and black would not be 

 colours, but merely stimulation or absence of stimulation of 

 the rods as wholes. 



The curious phenomenon of colour-blindness in individuals, 

 and perhaps also the assumed existence of racial deficiencies 

 in the matter of colour-sensation *, might be explained as due 

 to a failure to develop the necessary specialized gradations 

 in the sizes of the corpuscles suspended in and composing the 

 retinal rods. It is, on the other hand, possible that we have 

 not ourselves reached the limit of perfection attainable in this 

 direction. 



Jt may perhaps be added tliat, while the active force 

 causing the stimulation of the nerve-endings for the different 

 colours would in all cases be the same, namely pressure on 

 the sides of the rod by the crowded granules in the gangways, 

 the actual stimulus on the nerve-fibrils would be different for 

 each colour — in one zone of the rod the nerve-ends would be 

 nipped between larger, in another zone between smaller, 

 corpuscles, which also might perhaps differ in shape. 



We have, then, briefly traced a theory of light-sensation 

 and of vision which embraces all known eyes, from the 

 simplest to the most complex, and wiiich accounts for tlie 

 most perplexing phenomena by an appeal to known factors 

 alone. But, however connected and plausible a theory may 

 be, the question of most importance always is, can it be 

 proved ? Is it anything more than a mere working hypo- 

 thesis ? While admitting that this theory is, and may perhaps 

 long remain, only a working hypothesis, I think that it is 

 something more. A great part of it, dealing with the elemen- 



* The ancient Greeks, judging from the very limited colour vocabu- 

 lary of their poets, are thought not to have been so specialized in this 

 respect as we are to-day. 



