THE SENSATION OF SIGHT, 221 



It will be seen that this hypothesis is nothing more than a 

 further extension of Johannes Miiller's law of special sensation. 

 Just as the difference of sensation of light and warmth depends 

 dernonstrably upon whether the rays of the sun fall upon nerves 

 of sight or nerves of feeling, so it is supposed in Young's hypo- 

 thesis that the difference of sensation of colours depends simply 

 upon whether one or the other kind of nervous fibres are more 

 strongly affected. When all three kinds are equally excited, 

 the result is the sensation of white light. 



The phenomena that occur in red-blindness must be referred 

 to a condition in which the one kind of nerves, which are sensi- 

 tive to red rays, are incapable of excitation. It is possible 

 that this class of fibres are wanting, or at least very sparingly 

 distributed, along the edge of the retina, even in the normal 

 human eye. 



It must be confessed that both in men and in quadrupeds 

 we have at present no anatomical basis for this theory of colours ; 

 but Max Schultze has discovered a structure in birds and reptiles 

 which manifestly corresponds with what we should expect to 

 find. In the eyes of many of this group of animals there are 

 found among the rods of the retina a number which contain a 

 red drop of oil in their anterior end, that namely which is turned 

 towards the light ; while other rods contain a yellow drop, and 

 others none at all. Now there can be no doubt that red light 

 will reach the rods with a red drop much better than light of 

 any other colour, while yellow and green light, on the contrary, 

 will find easiest entrance to the rods with the yellow drop. 

 Blue light would be shut off almost completely from both, but 

 would affect the colourless rods all the more effectually. We 

 may therefore with great probability regard these rods as the 

 terminal organs of those nervous fibres which respectively 

 convey impressions of red, of yellow, and of blue light. 



I have myself subsequently found a similar hypothesis very 

 convenient and well fitted to explain in a most simple manner 



to J. J. Mttller's experiments (Archiv fur OphtJialmologie, XV. 2. p. 208^) 

 violet is more probable. The fluorescence of the retina is herd a source of 

 difficulty. 



