208 RECENT PROGRESS OF THE THEORY OF VISION. 



the optic nerve than other agents of an entirely different nature. 

 In one respect only does light differ from the other causes which 

 are capable of exciting this nerve : namely, that the retina, 

 being placed at the back of the firm globe of the eye, and further 

 protected by the bony orbit, is almost entirely withdrawn from 

 other exciting agents, and is thus only exceptionally affected by 

 them, while it is continually receiving the rays of light which 

 stream in upon it through the transparent media of the eye. 



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 noticeable 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 sensation 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 natu- 

 ral conneection puts it beyond doubt that external light is only 

 one of the exciting causes capable of bringing the optic nerve 

 into functional 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 exter- 

 nal objects. 



Light is known in Physics as a movement which is propa- 

 gated by successive waves in the elastic ether distributed 

 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 



