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them red or green, but both of them gray. The orchard or lawn would present the same appearance to their naked eyes as would its stereograph viewed in a stereoscope to the normal eye. For the iodized plate too is color-blind.

A comparison, again, of the eye and ear in this regard is instructive. The limits of perception of soundvibrations are very wide, viz., sixteen per second to more than thirty-two thousand per second, or about eleven octaves; the limits of perception of light-vibrations are very restricted, only about one octave. Now, in some ears the extreme limit is not perceived, but this is not considered a grave defect, for there is no special use for the extremest range. In the eye, too, the extreme limits, though so narrow, are sometimes not reached, but in this case the usefulness of the whole range makes this a serious defect. This is color-blind

In the ear the vibrations most commonly unperceived are at the upper end of the scale. In the eye the defect is usually at the lower and middle parts of the scale ; red or red and green are unperceived. The red-green blind see yellow and blue perfectly well.

But between the ear and eye there is this fundamental difference: In the case of the ear the defect of perception on the extreme limit of range is one not only of pitch, but also of sound; while in the case of the eye it is only of color, and not at all of light. This shows a probable essential difference in the nature of the response in the two cases.

Explanation of Color-blindness.—The severest test of the theories of color perception is their application in the explanation of color-blindness. The general theory of this defect is, that one or more of the normal ret

ness.

inal elements or retinal color-substances is wanting. Let us try the several theories by this test.

1. Helmholtz's Theory.—Perhaps the most commonly wanting of all colors is red. According to this theory the defect in these cases consists in the absence of the red color-substance. The normal eye is trichromic, the red-blind eye is dichromic. Now, it is certainly true that the color-blind eye is dichromic; but, as first pointed out by Pole, who is himself color-blind, the two colors seen are not usually green and blue or violet, as it should be on this theory, but yellow and blue or violet.* The most common of all forms of color-blindness is red-green blindness. These persons see yellow perfectly well. But if yellow be not a primary color, but a mixture of red and green, how is it that yellow is seen ? Again, according to this theory the sensation of white is due to the photo-chemical affection of all the three primary color-substances. How, then, can colorblind persons see white when one or more of its constituents are wanting ? † For that they do see white as normal eyes do, is proved by cases in which one eye only is color-blind. In such cases the two eyes used alternately see white exactly alike. The great objection to Helmholtz's theory, then, is the normal perception of yellow when both its constituents are wanting, and of white when one or more of its constituents are wanting.

2. Hering's Theory.—Once admit that perception can result from restitutive processes, and Hering's theory

* Nature, vol. xx, pp. 477, 611, 637, 1879 ; Contemporary Review, May, 1880.

+ The red-blind by this theory ought to sce white as a bluish green, as normal eyes do when the red substance is exhausted by gazing intently on a red spot and then turning the eyes on a white sheet. Under these conditions the normal eye is temporarily red-blind.

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of color perception explains the phenomena perfectly. According to him, in cases of red-green blindness (the commonest of all) the red-green substance is wanting, while the yellow-blue substance is present. It is inevitable according to this theory that complementaries should be wanting together. Accordingly we do find cases of yellow-blue blindness, although they are rare. The perception of white, of course, presents no difficulty, because, according to him, white and black are primary complementaries due to a peculiar substance which seems never to be wanting. The real objections to Hering's theory are of another kind, already mentioned.

3. Mrs. Franklin's theory explains the phenomena well. In the gradual evolution of the eye from earliest times and from lowest animals to its present perfected? condition, (1) first only gray substance was present, and therefore only white and black and all shades of gray were seen. This, as a primitive condition, is a priori almost certain. (2) Then this primary visual substance was differentiated into two color-substances, yellow and blue; and therefore these two colors, together with white and black and gray, were all that were seen. (3) Then, finally, the yellow color-substance was secondarily differentiated into red and green colorsubstances, which produce respectively these colors, but still by their combination may reconstitute yellow substance and produce the sensation of yellow in the normal eye, precisely as the combination of all may reconstitute gray substance and produce the sensation of white or gray. Now, according to Mrs. Franklin, colorblindness, like so many other defects in the animal body, is an example of atavism-i. e., a return to primitive conditions. Complete color-blindness (which

sometimes, though rarely, occurs), in which no colors of any kind are seen, but only white and black and shades of gray, is the result of complete atavism, or a return to Stage 1. Red-green blindness, the most common of all, is a return to Stage 2. Of course, Stage 3 is the normal and most common condition. There are other forms of color-blindness—for example, yellow-blue blindness—which can not well be explained by this theory, although easily by Hering's. But this form is very rare,

and
may

be a defect in the cortical substance of the brain.

In further justification of this view it may be urged (1) that the “law of differentiation” is the most universal law of biological evolution, and therefore it is almost certain that retinal structure and visual substance, like all else, is subject to this law. (2) That the evolution of the ear and the sense of hearing seems to have followed a course analogous to that attributed to the eye and the sense of sight. As in the evolution of the ear, the labyrinth (vestibular sac and sernicircular canals) was first developed, and then the cochlea, and therefore sound was perceived first only as noise, and then also as tone, so in the evolution of the eye the rods were first developed, and then the cones; and therefore light was perceived first as white and shades, and then as colors.

What the Color-blind really see.—We are now in position to explain what the color-blind really see. The completely color-blind see a landscape with all its colors of earth and sky precisely as the stereograph of the same landscape is seen in a stereoscope by the normal eye. He sees shades, but not color. But this case is

As there are various kinds and degrees of colorblindness, we will take only the most common kind, viz., red-green blindness. In persons affected with this too common defect, some colors are seen perfectly correctly, some incorrectly, and some not at all as colors, but as shades. Of pure colors, what they see at all they see correctly, the rest they see only as shades. The mixed colors they always see incorrectly. We give below a schedule showing what the red-green blind see.

rare.

It will be observed that in their colorscheme there is a great predominance of browns and slate-blue.

1. See correctly. a. White and black and all intermediate

shades or grays. 6. Yellow and all shades of the same, i. e.,

browns. c. Blue and all shades of same or slate-blues.

PURE COLORS.

11. Don't see at all as Colors.

a. Reds are geen as different shades of

gray. b. Greens are seen

different shades of gray.

MIXED COLORS.

III. See incorrectly. a. Scarlet = red and yellow: are seen as gray

and yellow = dark brown. 6. Orange = red and yellow : are seen as gray

and yellow == lighter brown. c. Purples = red and blue: are seen as gray

and blue = slate-blue. d. Yellowish green = yellow and green: are seen as yellow and gray = brown.

blue and green: are seen as blue and gray = slate-blue.

e. Bluish green

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