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for yellow in the above description of red, orange, yellow, and yellow-green, the brightest red would be called dark green, and they would fail to see at all in the extreme red, the spectrum being shortened. These latter would also recognize a white or grey band, but it would be in a position rather nearer the blue of the spectrum than in the first case (see No. 3, Plate I). It is needless to say that to normal vision this white or grey band is non-existent, and whenever a person under examination sees such a band the evidence is conclusive that he is colour-blind. These differing descriptions of the spectrum show that this form of colour-blindness may be divided into two classes, which for convenience sake may be termed green- and red-blindness. Another point of difference between them is the part of the spectrum that appears brightest. To the normal eye it is the yellow, and to the green-blind it is nearly at the same place, but to the redblind it is the green. This, perhaps, may give a clue to the designation of the spectrum colours by these two classes. To the green-blind, red and yellow are the same colour, but the yellow being the brighter he looks on red as degraded or darkened yellow. On the other hand, to the red-blind green is brighter than yellow or orange, and these appear as degraded green.

Experiment has shown that every colour in nature, as seen by a normal eye, can be expressed as a mixture of three, so that normal vision is tri-chromatic. In a similar sense the more pronounced types of ordinary colour-blind vision are di-chromatic. These colour relations must be regarded as purely subjective, for enough is now known of the nature of light to exclude the possibility of a three-fold physical constitution. In the theory of Young, subsequently, and independently, brought forward and developed by Helmholtz, light is supposed to be capable of exciting three distinct primary sensations, combined in varying proportions, and dependent upon the quality of the light. As to the character of the three sensations, Young identified them with red, green, and violet; and no widely-differing choice is possible, unless upon the supposition that the primary sensations, in their purity, are quite outside the range of our experience. The yellow of the spectrum, for example, cannot be primary, for it is capable of being matched by a suitable mixture of red and green. According to this view each primary sensation is excited in some degree by almost every ray of the spectrum; but the maxima occur at different places, and the stimulation in each case diminishes in both directions, as the position of maximum is receded from.

The Young. Helmholtz theory of colour-vision.

The following diagram will convey the idea of this theory:

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The lines with the letters B, C, D, &c., below the curves indicate certain fixed lines in the solar spectrum whose wave-lengths have been determined.

The different degrees of the stimulation given to each of the three sensations by every part of the spectrum is shown in the diagram by the heights of the curves above the horizontal base line. Thus in the middle of the spectrum, near E, each of the curves is to be found of a different height, and these degrees of stimulation of the three sensations, combined together, give the sensation of spectral green. It may be remarked that, on the scale adopted, the three sensations are supposed to be equally stimulated when white light is perceived. The areas of the three curves are therefore equal, and at the places in the spectrum where the curves are of the same height, the stimulation of the sensations is also the same. At the extreme red and extreme violet of the spectrum the curves of the red and violet sensations are alone to be found, hence at those parts the sensations are simple.

According to this theory, the two types of complete red-greenblindness are attributed to the absence of either the red, or else of the green sensation, the absence of the former corresponding to red-blindness, and of the latter to green-blindness. Where the violet and green curves cut, the red-blind person will see what to him is white, and where the red and violet curves cut the green-blind will also similarly describe his sensation of colour. To the normal eye these parts of the spectrum appear as bluish-green and green, as there is a stimulation of the green and violet sensations, or of the green alone, over and above that necessary to produce with the red sensation the mixed sensation of white.

In considering the question as to how far red-green blindness can be regarded as a mere deficiency in colour-perception, it is important to bear in mind that, according to recent observation, considerable deviations from the normal type may occur without any approach to colour-blindness. If we imagine a di-chromatic system be derived from an abnormal tri-chromatic system by the suppression of one sensation, it will differ from a di-chromatic system similarly derived from a normal system of colour-vision.

Blindness to violet, and shortening of the violet end of the Violet colourspectrum, have been described, but the instances are very few. blindness. One case of apparent violet-blindness of which the Committee have cognizance answers accurately to the Young-Helmholtz theory, on the supposition that the violet sensation is absent (see No. 4, Plate I).

Three other cases of congenital colour-blindness investigated by the Committee deserve special mention; two (brothers) in which there was but one sensation, answering probably to the violet sensation of the Young-Helmholtz theory, and the third in which the principal sensation was a pure green with perception of white and probably a slight trace of red. As these were all cases of congenital colour-blindness, they are mentioned as in some measure confirming the theory in question (see Note a).

Another theory, that of Hering, starts from the observation that Hering's when we examine our own sensations of light we find that theory of certain of these seem to be quite distinct in nature from each colour-vision. other, so that each is something sui generis, whereas we easily recognise all other colour sensations as various mixtures of these. Thus, the sensation of red and the sensation of yellow are to us quite distinct: we do not recognise anything common to the two; but orange is obviously a mixture of red and yellow. Green and blue are equally distinct from each other and from red and yellow, but in violet and purple we recognise a mixture of red and blue. White again is quite distinct from all the colours in the narrower sense of that word, and black which we must accept as a sensation, as an affection of consciousness, even if we regard it as the absence of sensation from the field of vision, is again distinct from everything else. Hence the sensations, caused by different kinds of light or by the absence of light, which thus appear to us distinct, and which we may speak of as "native" or "fundamental" sensations, are white, black, red, yellow, green, blue. Each of these seems to us to have nothing in common with any of the others, whereas in all other colours we can recognise a mixture of two or more of these.

This result of common experience suggests the idea that these fundamental sensations are the primary sensations, concerning which we are inquiring. And Hering's theory attempts to reconcile, in some such way as follows, the various facts of colour-vision with the supposition that we possess these six fundamental sensations. The six sensations readily fall into three pairs, the members of each pair having analogous relations

to each other. In each pair the one colour is complementary to the other; white to black, red to green, and yellow to blue.

Now, in the chemical changes undergone by living substances, we may recognise two main phases, an upward constructive phase in which matter previously not living becomes living, and a downward destructive phase in which living matter breaks down into dead or less living matter. Adopting this view we may, on the one hand, suppose that rays of light, differing in their wave-length, may affect the chemical changes of the visual substance in different ways, some promoting constructive changes (changes of assimilation), others promoting destructive changes (changes of dissimilation); and on the other hand, that the different changes in the visual substance may give rise to different sensations.

We may, for instance, suppose that there exists in the retina a visual substance of such a kind that when rays of light of certain wave-lengths-the longer ones for instance of the red side of the spectrum-fall upon it, dissimilative changes are induced or encouraged, while assimilative changes are similarly promoted by the incidence of rays of other wave-lengths, the shorter ones of the blue side. But, it must be remembered, that in dealing with sensations it is difficult to determine what part of the apparatus causes them; we may accordingly extend the above view to the whole visual apparatus, central as well as peripheral, and suppose that when rays of a certain wave-length fall upon the retina, they in some way or other, in some part or other of the visual apparatus, induce or promote dissimilative changes and so give rise to a sensation of a certain kind, while rays of another wave-length similarly induce or promote assimilative changes and so give rise to a sensation of a different kind.

The hypothesis of Hering applies this view to the six fundamental sensations spoken of above, and supposes that each of the three pairs is the outcome of a particular set of dissimilative and assimilative changes. It supposes the existence of what we may call a red-green visual substance, of such a nature that so long as dissimilative and assimilative changes are in equilibrium, we experience no sensation, but that when dissimilative changes are increased, we experience a sensation of (fundamental) red, and when assimilative changes are increased we experience a sensation of (fundamental) green. A similar yellow-blue visual substance is supposed to furnish, through dissimilative changes, a yellow, through assimilative changes a blue sensation; and a white black visual substance similarly provides for a dissimilative sensation of white and an assimilative sensation of black. The two members of each pair are therefore not only complementary but also antagonistic. Further, these substances are supposed to be of such a kind that while the white-black substance is influenced in the same way, though in different degrees, by rays along the whole range of the spectrum, the two other substances are differently influenced by rays of different wave-length. Thus, in the part of

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