originators of nerve-force. They evidently have an important function. They probably act as little nervecenters; and many unconscious, involuntary, or reflex acts of vision are probably performed by their means, without referring the sensation to the brain.

The manner in which the whole apparatus operates is briefly as follows: The light penetrates through the retina until it reaches the outer layer of rods and cones. These are specially organized to respond to or co-vibrate with the undulations of light. These vibrations are carried through the connecting threads to the fibrous layer, then through the fibers of this layer to the optic nerve, then along the fibers of the optic nerve to the gray matter of the brain, where they finally determine changes which emerge into consciousness as the sensation of light.

That we have correctly interpreted the function of the layer of rods and cones is rendered probable not only by its very remarkable and complex structure, adapting it to responsive vibrations, but also by the peculiar properties of two spots on the retina on which all the layers do not co-exist. Just where the optic nerve enters, as shown in Fig. 21, page 55, the bacillary layer is necessarily wanting, and it is the only spot in which this is the case. Now, this spot is blind (see page 78). Again, just in the axis of the globe, or what might be called the south pole of the eye, is the central spot or central pit. In this spot is wanting the fibrous layer and the whole of the middle layer, except the multipolar cells. The bacillary layer is here, therefore, directly exposed to the action of light. Now, this is the most sensitive spot of the retina.

Perception of Color.-Color, like musical pitch, consists of an infinite number of kinds and shades; but

these may be reduced to a few primary kinds, by the mixture of which the intermediate shades may be supposed to be made. Newton made seven primary colors in the solar spectrum; but though these, and indeed many more, may be considered distinct from the physical point of view, since they are the result of different rates of ethereal vibration, yet they can not be all considered as primarily distinct sensations. Brewster reduced all color-sensations to three primary, viz., red, yellow, and blue. Young made them red, green, and violet. This latter view is adopted by Helmholtz and most modern writers.

Recently, however, Hering * has reinvestigated the whole subject with great acuteness, from the purely physiological instead of physical point of view, and arrives at different results. Hering includes white and black among his primary color-sensations, making six in all. But, leaving out these as belonging rather to the category of shades or nuances, according to Hering there are four and only four primary color-sensations essentially distinct from each other, viz., red, yellow, green, and blue. Aside from all physical considerations, undoubtedly this is true. These four colors are essentially distinct and irresolvable into any mixture of others. Again, according to Hering, these four are reducible to two complementary pairs, viz., red and green on the one hand, and yellow and blue on the other. This is also undoubtedly true. Finally, according to Hering, complementary colors are the result of opposite affections of the retina, so that there are only two essentially distinct color-affections of the retina, which, with their opposites, produce the two pairs of complementary colors: the one with its opposite produces red and green;

* Hering, "Zur Lehre von Licht-Sinne," Wien, 1878.

the other with its opposite, yellow and blue. This, though more doubtful, seems a probable cause of complementariness.

Theory of Color-Perception.-Color-perception is undoubtedly a simple perception, and irresolvable into any other. It must, therefore, have its basis in retinal structure. Since light is perceived by co-vibration of retinal elements, and since the different colors have different rates of vibration, there must be a corresponding structure of the retinal elements, by means of which they co-vibrate with each of these colors. In the ear different rates of aërial vibration (musical pitch) are perceived by means of rods of different lengths (rods of Corti), which co-vibrate, each with its own pitch. It seems probable, therefore, that different rods or cones covibrate with different rates of ethereal undulations, i. e., with different colors. This is the commonly received view, brought forward first by Young. It is supposed that there are three kinds of rods or cones, which severally co-vibrate with the three primary colors of Young. One kind responds to the slower vibrations of red, another kind to those of green, and still another to the more rapid vibrations of violet. When two kinds vibrate, intermediate colors are perceived. When all vibrate together, then white light is perceived. Or, to express it differently, intermediate colors produce vibration of two kinds, white light of all kinds, of rods. Or, if we adopt the theory of Hering in regard to the primary colors, one kind of rod or cone responds to red and green, another kind to yellow and blue.

Very recently Stanly Hall has proposed a theory which seems even more probable.* He believes that color is perceived by the cones alone; further, that

* "American Academy of Science and Art," vol. xiii, p. 402 (1878).

different parts of the same cone vibrate with different degrees of rapidity, and therefore respond to different colors, and that the conical form is adapted for this purpose. In order to gain clearer conception, we may imagine each cone to be made up of a number of buttons of graduated sizes joined together. These buttons, on account of their different sizes, would vibrate with different degrees of rapidity, and therefore co-vibrate with different colors. White light, he supposes, vibrates the whole series; red light, the thicker, and violet, the thinner, portion of the series; or, taking Hering's view of the primary colors, we may imagine that red and green rays affect one portion, and yellow and blue rays another portion, of the same cone.

The subject of the mechanism of color perception, however, is yet in the region of speculation, though probably of profitable speculation. To pursue it any further would be unsuited to the character of this treatise.

Daltonism, or Color-Blindness.-Many persons lack a nice discrimination of shades of color. Such persons see colors perfectly well, but, from want of attention or culture, have not learned to nicely discriminate and name them. This must not be confounded with colorblindness. The color-blind do not see some colors as colors at all. The defect is not one of culture, but of sensation. We can best explain it by comparing the eye and ear.

The limits of the perception of sound-vibrations are very wide, viz., more than eleven octaves. The limits of perception of light-vibrations are far more restricted, viz., only a little more than one octave. Now in many ears the extreme limits are not perceived; but this is not considered a defect, because there is no special use for

the extremest range. So in the eye. Even the narrow limits of the normal eye are sometimes not reached; but in this case the usefulness of the whole range makes it a serious defect. This is color-blindness. In the ear the vibrations most commonly unperceived are at the upper end of the scale. In the eye it is usually the lower end of the scale which is defective, viz., red, or red and green. The color-blind see yellow and blue, but not red and green.

This defect was first brought to scientific notice by the celebrated chemist Dalton, and after him has often been called Daltonism. The peculiarities of Dalton's vision were carefully investigated by Sir John Herschel, and the first scientific explanation was given by him. Adopting the view of Young of three primary colors, Herschel regarded normal vision as trichromic, but the vision of Dalton as dichromic, the red being wanting. This view certainly explained the most striking phenomena of color-blindness, but it does not explain the fact that green is wanting as well as red. As shown by Pole* (who is himself color-blind), the phenomena are far more perfectly explained on Hering's view of the primary colors; and conversely, the phenomena of colorblindness are a powerful argument in favor of Hering's view. Of the two pairs of complementary colors of Hering, one pair, viz., the red-green, is wanting in the color-blind, while the other pair, yellow-blue, is perceived as in normal vision. The colors and shades, therefore, which are perceived by the color-blind are: 1, black and white, and all intermediate shades of gray; 2, yellow in all its shades; and, 3, blue in all its shades. A pure red seems to them a dark gray; but if mixed with yellow, as

* "Nature," 20, pp. 477, 611, 637 (1879); "Contemporary Review," May, 1880.

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