94 THE POPULAR SCIENCE MONTHLY. 



produced will have its basis, not in complexity of tissue, but in the 

 varying action of the affecti7ig agent. 



"Without entering into a discussion of the question in detail, I 

 would say that it seems probable that the optic nerve is merely a 

 highly organized nerve of common sensation. In some of the lower 

 forms of animal life light is perceived over the whole cutaneous or 

 external surface, as shown by the action of the animals when exposed 

 to its influence. Furthermore, it is now a generally admitted fact 

 that heat and light are due to vibrations of the same ether, differ- 

 ing only in their wave-lengths. The effect of both heat and light is 

 to produce molecular change. When heat produces a sensation through 

 the cutaneous nerves, it is most probable that it does it by means of a 

 molecular change in the terminal filaments of these nerves which is 

 communicated to the brain-center through the nerves, probably also 

 by a rapidly progressive change in their molecular structure. The 

 nerves of common sensation, however, do not seem to possess the power 

 to differentiate variations in wave-lengths they take cognizance only 

 of the varying intensity of the vibratory motion ; that is to say, they 

 distinguish quantities rather than qualities. It would, however, be 

 doing no violence to known facts to suppose that a high specialization 

 would enable these nerves to carry as distinct impressions the changes 

 wrought by the separate wave-lengths. In fact, it is highly probable 

 that they do so, but the cerebral centers in which they terminate have 

 not been educated to the point of making distinctions between these 

 separate impressions and fixing them as individual sensations. 



In framing a theory of color-perception on the basis we have indi- 

 cated, we would suppose the retina to be a body whose molecular 

 structure is such that it will respond with promptness to all or nearly 

 all the wave-lengths of perceptible light. This molecular change pro- 

 duced in the retina is carried by the optic nerve to the center of vis- 

 ion in the brain, and is there converted into a sensation. This is, to 

 some extent, going back to the original theory of Newton, who, in 

 speaking of the action of light upon the retina, considered that "the 

 rays impinging upon the ends of the optic nerve excite vibrations 

 which run through the optic nerve to the sensorium. Here they are 

 supposed to affect the sense with various colors according to their nat- 

 ure and bigness." 



The chief objection to this hypothesis, advanced by Young, was 

 that the frequency of these vibrations must be dependent upon the 

 constitution of the substance of the retina, and it was almost impossi- 

 ble that every sensitive point should have an infinite number of differ- 

 ent particles to respond to this infinite number of vibrations. He there- 

 fore supposed the number to be limited to three which corresponded 

 to red, green, and violet. 



It will be seen that the difference in the different theories of colors 

 lies in the supposed reaction of the retina to light. After the impres- 



