produced will have its basis, not in complexity of tissue, but in the varying action of the affecting 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, differing 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 indicated, 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 produced in the retina is carried by the optic nerve to the center of vision 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 nature 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 impossible that every sensitive point should have an infinite number of different particles to respond to this infinite number of vibrations. He therefore 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-