Where (i. e., through the bacillar layer) the optic nerve enters the retina, the eye is blind. This seems to prove that the bacillary layer is necessary to sight. But this layer contains the rods and cones. These are said in Foster’s “Physiology” to be transparent, refractive, doubly refractive, and very sensitive to light, changing in size in different degrees of it. Possibly they may act in some way analogously to prisms. But however they may act, Fig. 131 shows that the rods because smaller should be more sensitive to slight vibratory effects than the cones; and Fig. 128 shows that the central spot, which sees outlines and colors the most distinctly, contains only cones. Are the rods, therefore, affected, according to what was said on page 379, by light in general, and the cones by local color in particular objects? Again, each rod and cone possesses two apparently separated limbs, the larger of which is nearer the main body of the nerves than the smaller. If a wave of white light affect each limb similarly, this wave divided and changed in form, as when color is produced, must affect each differently. In this case is one cone-limb affected by the principal color-wave, and the smaller, with reflecting rods near it, by the twin complementary color-wave? All around the rods and cones, and inside the former, a purplish-blue liquid is constantly advancing and receding. It has been supposed that the sole purpose of this is to record different degrees of light and shade. But, while recording these, it may do very much more. Most of us must have noticed, when the power is turned from an electric light, that the one platinum wire vibrating at different rates produces all the warm colors white-yellow, yellow, orange, and red; and it is a fact easily shown that these colors respectively, when shining through blue glass, produce all the cold colors, blue, green olive-green, and purple. The attributing of articulative sounds to different rates and forms of vibrations when affecting the same ossicles in the ear suggested to Professor Bell that apparatus for converting the vibrations in an electric wire into sounds which made the telephone a success. Why is it not reasonable to suppose that the same rods or cones, when vibrating differently, shaded or not by blue, can produce all the colors, so that the mind can see them as well as the outlines in the picture impressed upon the retina. Another thought : vibrations of particles of matter against one another or the air usually generate heat. Heat thus generated usually generates chemical action. Different rates of vibrationand this is why, as has been proved, it is true of different colorsgenerate different degrees of heat and of chemical action. Chemical action, so scientists tell us, manifested in the pulling down and building up of tissue,
is the method through which the nerves communicate sensations. What then? The author is aware that he has suggested an explanation of the way in which sound-waves or sight-waves may affect the organs of the ear or eye, and through them the nerves and the mind back of them, which is not in the books. But can any explanation be found in them as plausible, or as free from objections, as is this one? Certainly it is not any explanation ascribing the recognition of any pitch or color to a separate organ fitted to respond sympathetically to it and to it alone. So far, at least, as concerns the organism, as represented in Figs. 130 and 131, there is no reason to suppose otherwise than that all the rods and cones may be equally fitted to respond to the waves of light of any color, and yet with different degrees of susceptibility, some possibly the rodsrepresenting only atmospheric light and color, and somepossibly the conesthat color which appears in particular objects.
The explanation thus suggested not only refers to a similar cause the subjective effects both in the ear and the eye . . . but it seems to explain also the most important difference between the effects of successive’ and of simultaneous 2 contrast. This is that the time of the continuance and the brilliancy of a color in successive contrast depend upon the length and strength of the vibratory condition preceding it, whereas, in simultaneous contrast, such effects depend neither upon the length of time during which one looks at a color, nor even upon its comparative fulness. This difference is exactly what, according to our hypothesis, we should expect. According to it, the continuance and character of the oscillations occasioning successive contrast will, of course, be determined by the quantity and quality of their previous excitation. On the contrary, the complementary color produced in simultaneous contrast depends upon the presence by its side of the local color, and it is neither in-creased nor lessened in intensity by its continued presence. Moreover, in every place where this complementary hue can become visible, there is already some other shade or tint with which its hue must blend, and . . . produce a mixed, and therefore never, save in very exceptional cases, a brilliant effect.