236 RECENT PROGRESS OF THE THEORY OF VISION. 



On the other hand, the optic nerve, by reason of the 

 peculiar structures in connection with the ends of its 

 fibres, the rods and cones of the retina, is incomparably 

 more sensitive to rays of light than any other nervous 

 apparatus of the body, since the rest can only be affected 

 by rays which are concentrated enough to produce notice- 

 able elevation of temperature. 



This explains why the sensations of the optic nerve are 

 for us the ordinary sensible sign of the presence of light 

 in the field of vision, and why we always connect the sen- 

 sation of light with light itself, even where they are really 

 unconnected. But we must never forget that a survey of 

 all the facts in their natural connection puts it beyond 

 doubt that external light is only one of the exciting 

 causes capable of bringing the optic nerve into func- 

 tional activity, and therefore that there is no exclusive 

 relation between the sensation of light and light itself. 



Now that we have considered the action of excitants 

 upon the optic nerve in general, we will proceed to the 

 qualitative differences of the sensation of light, that is 

 to say, to the various sensations of colour. We will try to 

 ascertain how far these differences of sensation correspond 

 to actual differences in external objects. 



Light is known in Physics as a movement which is 

 propagated by successive waves in the elastic ether distri- 

 buted through the universe, a movement of the same kind 

 as the circles which spread upon the smooth surface of a 

 pond when a stone falls on it, or the vibration which is 

 transmitted through our atmosphere as sound. The chief 

 difference is, that the rate with which light spreads, and 

 the rapidity of movement of the minute particles which 

 form the waves of ether, are both enormously greater than 

 that of the waves of water or of air. The waves of light 

 sent forth from the sun differ exceedingly in size, just as 



