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This is the simplest conception of the process; but it is now far more probable that it is due rather to a photochemical change in a peculiar substance or peculiar substances, which we may call color-substances in the cones. As the iodized plate, so these color-substances in the retina are differently affected by light of different rates of vibration.

The general theory given above is universally accepted; but when we attempt to express more definitely the physical correspondent of the perception of different colors, then our theory becomes more hypothetical. There are several such special theories. They are acceptable in proportion as they explain the phenomena.

Young-Helmholtz Theory.-According to the YoungHelmholtz theory of three primary colors, there are three distinct kinds of retinal cones, which respond rerespectively to three rates of ethereal vibration, and give rise to the perception of the three primary colors. If the vibrations are of such rate as to find response in only one kind of cone, we have pure color; but if of intermediate rates so as to affect two kinds we have mixed colors; if they affect equally all kinds, we have white. Or else we may say that there are three color-substances, each photo-chemically sensitive to one of the three primary colors, but when two kinds or all kinds are affected, they give rise to mixed colors or to white of various shades or grays.

Hering's Theory.-Leaving out, for reasons already given, white and black from the category of colors, according to Hering, in the retinal cones are found two kinds of color-substance, each of which is photo-chemically affected in two opposite ways, viz., by decompo

* Observe, not colored substances but color-substances, i. e., substances which by photo-chemical change produce the sensation of color.

sition and recomposition-by destruction and restitution, by katabolism and anabolism. These two color-substances by opposite affections give rise to two pairs of complementaries, one to red-green and the other to yellow-blue; and the essential nature of complementariness, especially their mutual destructiveness, is thus easily explained. This accords well also with the artist view of colors embodied in the terms warm

FIG. 34.

[graphic]

Y

Gr

R

Or

DIAGRAM TO ILLUSTRATE HERING'S THEORY OF COLOR VISION.-R G = redgreen; YB yellow-blue, strong line; WH= white. (After Foster.)

(red and yellow) and cool (green and blue), the one more fatiguing because destructive, the other more restful because restitutive. Fig. 34, taken from Foster, is an attempt to graphically represent Hering's view. The horizontal line n n represents the extent of the visible spectrum. The places of the spectral colors are represented by the vertical lines from the letters R, Or, Y, Gr, Bl, V. The vertically lined space represents the affections of the red-green substance, and the horizontally lined space the affections of the yellow-blue substance. In each the space above the horizontal line nn shows katabolism or positive work or decomposi

tion; and the space below, anabolism or negative work or restitution. The strong line wh wh shows the distribution of light irrespective of color, i. e., white light throughout the spectrum. This line is supposed to

show the affections of the so-called white-black substance, but it can express only katabolic and not anabolic changes, and therefore white only, not black, except as absence of light.

Mrs. Franklin's Theory. Mrs. Franklin has recently* brought forward a theory which has deservedly attracted much attention. According to her, there are insuperable objections to both the current theories. The objection to Helmholtz's theory is its failure to explain the phenomena of color-blindness, as will be shown in connection with that subject. The objection to Hering's theory is that some of its suppositions are in conflict with the most fundamental principles of physiology. According to Hering, the complementary pair red-green is the result of opposite processes, destructive and constructive, katabolic and anabolic, in the same color-substance; and so also of the pair yellow-blue. Therefore activity or energy (for surely there is some energy expended in the perception of green or blue) may be generated by re-composition, reconstruction, anabolism. But it is a fundamental principle in physiology that vital energy is produced always at the expense of tissue-is generated always by katabolism. Constructive work does not and can not create but only consume, can not set free but only absorb energy. Negative energy seems a contradiction in terms. To the physiologist this will seem a fatal objection. But perhaps the psychologist may ask:

"Zeitschrift für Psych. und Physiol. der Sinnesorgane," B. J. IV, 1892. "Mird," vol. ii, p. 473.

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"May there not be definite states of consciousness corresponding also to restitutive processes? Are states of consciousness necessarily associated with expenditure of energy?" So much for the objections to other theories. Now her own.

*

Mrs. Franklin supposes that there exists in all parts of the retina a fundamental visual substance which by photo-chemical change affects the retina in such wise as to produce the sensation of white of all shades, and which therefore may be called gray substance. This is always present in all parts of the retina, and in the history of the evolution of the eye was, at first, the only one. In the cones, but not in the rods, some of this substance is differentiated at first into two color-substances, yellow and blue, and finally into three colorsubstances, red, green, and blue, some of the yellow substance having been secondarily differentiated into red and green substances. Mixed colors, as in other theories, are due to simultaneous affections of two or more color-substances in varying proportions. Sunlight decomposes all in proportions exactly corresponding to the composition of the gray substances, and therefore produces the same sensation-in fact, may be said to reconstitute the gray substances. Yellow light decomposes the red and green substance in proportions corresponding to the original composition of the yellow substance; in fact, may be said to reconstitute the yellow substance, and therefore produces the same sensa-` tion. Thus the perception of white or gray may be due either to photo-chemical change in undifferentiated gray substance, as is doubtless the case in the rods, or to decomposition of all the color substances in propor

*I give Mrs. Franklin's theory substantially as I understand it. In the attempt to make it clear, I have left out many details.

tions reconstituting gray substances, as is probable in the cones. Similarly the perception of yellow may be due either to photo-chemical change of some undifferentiated yellow substance, or else to the decomposition of the red and green color-substances in proportions reconstituting yellow substance. Both white or gray and yellow would on this view be primary sensations, because they were, and still are largely due to the decomposition of original substances.

Color-blindness.

The defects of the eye already treated in Chapter II, Section II, are defects of the image-forming instrument; color-blindness is a defect of the receiving plate, a defect of retinal structure. As before we treated first the structure of the normal instrument, and then of its defects, so now, having given the supposed normal retinal structure, we come to treat of its defects.

What is Color-blindness ?—Many persons lack a nice discrimination of colors and their shades. Such persons may see colors perfectly well, but from want of attention and culture, and especially for want of any accepted standard of colors and their names, have not learned to discriminate and name them. This must not be confounded with color-blindness. The color-blind do not see some colors at all as colors, but only as shades. The defect is not one of culture but of sensation, and therefore of retinal structure. An example will make this plain. In the commonest form of this defect, the sensations of red and green are wanting. To such a person the bright-green leaves and brightred berries of a cherry orchard in full fruit, or red flowers and the green lawn on which they grow, would ceem nearly or quite of the same tint, and neither of

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