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The YoungHelmholtz theory of colour-vision.

The following diagram will convey the idea of this theory:

[graphic]
[table]

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The lines with the letters B, C, D, 4c., below the curves indicate certain fixed lines in the solar Bpectrum 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 fouud of a different height, and the»e degrees o,f 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 greeu 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 nalure 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 changesr 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 the spectrum which we call red, the rays rromote great dissimilative changes of the red-green substance with comparatively slight effect on the yellow-blue substance; hence our sensation of red.

[graphic]

The vertical shading represents the red and green, and the horizontal shading the yellow and blue, antagonistic pairs of sensations. The thick line indicates the curve of the white sensation.

In that part of the spectrum which we call yellow the rays effect great dissimilative changesof the yellow-blue substance, but their action on the red-green substance does not lead to an excess of either dissimilation or assimilation, this substance being neutral to them; hence our sensation of yellow. The green rays, again, promote assimilation of the red-green substance, leaving the assimilation of the yellow-blue substance equal to its dissimilation; and similarly blue rays cause assimilation of the yellowblue substance, and leave the red-green substance neutral. Finally, at the extreme blue end of the spectrum, the rays once more provoke dissimilation of the red-green substance, and by adding red to blue give violet. When orange rays fall on the retina, there is an excess of dissimilation of both the red-green and the yellow-blue substance; when greenish-blue rays are perceived there is an excess of assimilation of both these substances; and other intermediate hues correspond to varying degrees of dissimilation or assimilation of the several visual substances.

When all the rays together fall on the retina, the red-green and yellow-blue substances remain in equilibrium, but the whiteblack substance undergoes great changes of dissimilation; and we say the light is white.

According to this theory what are called red and green blindness are identical. The yellow-blue and white-black sensations remain, but the red-green sensation is absent in both. The white Colour-blindness caused by disease.

Statistics of colour-blindness.

or grey seen in the spectrum would then be due to the whiteblack sensation, as it alone is stimulated at that point.* (See Note b.)

The kinds of colour-blindness so far alluded to are the congenital types, but there is another form of colour-blindness which is induced by disease or injury. The former is apparently by far the most common, and so far as we have ascertained, is incurable, but the latter may be induced at any period of life, and in very many cases is capable of improvement or cure.

Colour-blindness induced by disease or injury exhibits dis tinctive features of its own, which are not present in cases of congenital colour-blindness. It is usually confined to the central region of the retina, and the extent of the diseased area varies largely. Defective form-vision is an invariable accompaniment, and it can be usually diagnosed by the recognized tests. (For these tests see Appendix VI.) In several cases induced by excessive use of tobacco, as also in that induced by progressive atrophy of the optic nerve, the Committee have found in examinations made with the spectrum that the sensations of white and blue alone were perceived in the central portions of the retina. The blue seen corresponded with the blue region of the spectrum, and all other colours were described as white. In other cases, a faint yellow in the yellow portion of the spectrum was perceived together with the blue and white, as in the first-named cases (see No. 5, Plate I). That these sensations were rightlj' described is to be assumed from the fact that these persons when in health have normal vision, and also, that on healthy portions of the retina all colours stimulate the normal sensations. (See Appendix C.)

The earlier statistics of defective colour-sense must be dismissed as untrustworthy, having bean arrived at by various, and frequently by inaccurate methods of examination, and having, on the whole, a marked tendency to error in the direction of excess. The first on which reliance can be placed are probably those of Dr. Joy Jeffries, of Boston, U.S.A., who personally examined 19,183 male persons, mostly in educational institutions, and who found among them 802 colour-blind, or 4-12 per cent. Among 14,764 females, he found only 11 cases, or 0'0084 per cent. In 1880, the Ophthalmological Society of London appointed a Committee to inquire into the subject, and they found that amongst 14,846 males, 617 or 4-16 percent. were colour-blind. Amongst 489 females, 0-4 per cent. were defective in colour-vision. The report of this Committee is contained in the first volume of the "Transactions" of the Society, and an extract from it will be found in Appendix I.

The Committee were furnished with some statistics regarding colour-blindness in two Japanese regiments. Out of 1,200 men examined, 19 were red-blind, 10 green-blind, 12 incompletely colour-blind, and 27 had weak colour-vision. This gives 3-4

* Without deciding between these two theories, it has been found convenient to accept the terminology of the Young-Helmholtz theory.

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