Two bright circles are seen, one on each side of the point of sight. One belongs to each eye. Which belongs to which? The answer to this question belongs to binocular vision; but we will say now in passing that since the entrance of the optic nerve is on the nasal side of the central spot, and since, as we shall see later, (page 116), impressions on the two nasal halves of the retinæ produce homonymously double images, in this case the bright circle on the right of the point of sight belongs to the right eye and that on the left to the left eye always. I find, in looking to the right, the left circle, and to the left, the right circle is the more brilliant.

SECTION III.-COLOR PERCEPTION.

Thus far we have spoken of the perception of light so far as concerns brightness or intensity and direction. We come now to speak of the perception of light as color.

Intensity versus Color.-As there are two kinds of perception of sound-viz., simple sound or sound as noise, loud or faint, and sound as tone or musical pitch, high or low-so there are two kinds of perception of light—viz., light as intensity or brightness or shade, and light as color. In both sound and light, the one is a question of strength of vibration or wave-height, the other of rate of vibration or wave-length. The range of perceptible vibrations in the case of hearing or tones is very great, from ten to eleven octaves-i. e., from sixteen per second to thirty-two thousand per second; in the case of light or color only about one octave, for

the rate of vibration of an extreme violet ray is only about double that of an extreme red.

Primary versus Secondary Colors.—Again, we must distinguish between pure or primary colors and mixed or secondary colors. The primary colors are those which can not be made by any mixture of others, and are few in number. Secondary colors are such as can be so made, and are infinite in number. Again, pure colors may be mixed not only with one another, but also in all proportions with white and with black. The former mixtures have been called tints, the latter shades, or else all may be called shades. There is some difference of opinion as to which, and how many, cofors should be called primary. This depends, partly at least, on the point of view, whether physical or physiological. Brewster made three primary colors-viz., red, yellow, and blue-regarding green as a mixture. Young and Helmholtz, and most physicists, make also three; but they are red, green, and violet, regarding yellow as a mixture. Brewster rejected green because of the wellknown fact that purest pigment-blue mixed with purest pigment-yellow makes a fine green. True enough; but the superposition of the yellow of the spectrum on the blue of the spectrum does not make green. On the contrary, they kill one another and make a gray. This is really the true test, for pigments are never pure colors. Both chrome yellow and ultramarine blue contain green. When they are mixed, the yellow and blue kill one another, and the green of both comes out. On the other hand, all the later physicists rightly reject yellow because this color is made by the superposition of spectral red and spectral green. From the physical point of view, therefore, green and not yellow is a primary color. From this point of view the three primary

colors may be regarded as spread out, each over the whole spectrum, but in greatest abundance the red at one end, the violet at the other, and the green in the

DIAGRAM SHOWING THE DISTRIBUTION IN THE SPECTRUM OF THE THREE PRIMARY COLORS OF YOUNG AND HELMHOLTZ.

middle, as shown in the diagram (Fig. 33).* The intermediate colors as seen are mixtures by overlap.

Hering takes up the subject from a wholly different point of view-physiological instead of physical. He investigates colors as sensations without reference to any physical considerations. From this point of view he makes six primary color-sensations essentially distinct from one another-viz., white, black, red, yellow, green, and blue. Or, if we relegate white and black to the category of shades instead of colors of intensity instead of quality, for which we will give reasons hereafter then by Hering's view there are four primary color-sensations-viz., red, yellow, green, and blue. Now, it can not be denied that from the pure point of view of sensation, unplagued by any physical considerations, Hering is right. Red, yellow, green, and blue are certainly perfectly distinct color-sensations irresolvable into any others or mixture of others, and they are the only colors thus irresolvable. This was recognized

* Helmholtz, in his latest utterances, adopts extreme blue instead of violet as the upper primary. (Stevens, vice-president's, address, p. 19, A. A. A. S., 1895.) IIis three primaries are carmine red, yellowish green, and ultramarine blue.

long ago by Leonardo da Vinci.* In orange and scarlet we distinctly see both red and yellow; in purple we see blue and red; and even in violet, one of the primaries of the physicists, we see distinctly blue with a glow of red. Further, Hering draws attention to the fact that his primaries consist of two pairs (or three pairs if we include, as he does, white and black) of complementaries which by mixture destroy one another-viz., red-green and yellow-blue. The importance of this in Hering's theory will be seen hereafter. We have taken white and black out of the category of colors. Hering is undoubtedly right in regarding these as distinct sensations, irresolvable into any other or mixture of others, but not as color-sensations. Black to the physicist is a negation of light, but it is a very positive sensation to consciousness and entirely different from darkness; so also white is a perfectly pure sensation. We indeed know that, physically, white is produced by a mixture of all the spectral colors, but we do not see these in white. But, although it is indeed true that white and black are pure sensations, yet I do not think that color is the proper word for them. As a mixture of all rates of aërial vibrations produces noise, not musical tone, so a mixture of all rates of ethereal vibrations produces white, not color. Therefore it is best to put white and black out of the category of colors into that of intensity or shades; and from this point of view, since shades are of every grade, we may speak of all shades from white to black as one sensation-viz., gray.

Theories of Color Perception.-General Account.-1. The perception of color is a simple perception, incapable of analysis, and therefore is doubtless connected *"Science," vol. i, p. 472, 1895.

with retinal structure of some sort. 2. Further, there is much reason to believe that it is an endowment of the cones, but not of the rods that the rods perceive light only as light or intensity, not as quality; or more specifically that the rods perceive white and black and all shades of gray between, but not colors. The cones perceive all shades also, but, in addition, colors. The reason for believing so is as follows: As already said (page 55), the bacillary layer in the central spot consists of cones only, and in going thence outward in all directions the cones become less and less numerous among the rods until at the anterior margin of the retina there are no cones at all, but only rods. Now, as the representative of these facts in the field of view, we find that the perception of color is most perfect at the point of sight, and becomes less and less so as we go outward in all directions, until, on the extreme margins of the field of view, it is wholly wanting. In other words, the distribution of color perception in the field of view corresponds perfectly to the distribution of the cones in the retina.

Again, 3, it is further believed that color is perceived by means of some kind of physical response to light-vibrations of different rates, and the simplest conception, and that which was first adopted, is of responsive vibration on the part of the cones of the retina. Musical pitch is perceived by responsive vibrations of the rods of Corti, which have graduated lengths like the strings of a piano, adapted to co-vibrate, each with its own pitch. So it has been supposed that different cones, or possibly, as suggested by Stanly Hall, different parts of the same cone, are structurally adapted to co-vibrate with different rates of ethereal vibration, and give rise to different sensations of color.