the spectrum which we call red, the rays promote great dissimilative changes of the red-green substance with comparatively slight effect on the yellow-blue substance; hence our sensation of red.

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 changes of 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

ness.

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 rightly 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 discolour-blind- missed as untrustworthy, having been 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 per cent. 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.

66

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 34

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

per cent. of soldiers who were colour-defective, without includ-
ing those who are classed as having weak colour-vision. The
above statistics all point to the prevalence of colour-blindness
amongst the male population, and to the fact that such defects
are not confined to one nationality or race.
The small per-
centage of colour-blindness found amongst women is remark-
able, but as it does not enter into the questions on which the
Committee have to report, it need not be further dwelt upon.

blindness.

The Committee have already briefly alluded to the mistakes Results and which congenitally colour-blind people are likely to make; but in dangers of order to emphasize it, they will enter rather more fully into the coloursubject. In the first place, let it be remembered that to the redblind and to the green-blind there is one green in the spectrum which they cannot distinguish from white, and which for convenience may be designated as their neutral colour. On the one side of this neutral band they see but green or red, more or less diluted with their neutral colour, and on the other side blue, also similarly diluted. The dilution increases as the neutral point is approached, and for some little distance on each side of it (unless a comparison with white be at hand) the dilution is so large that the colour may be mistaken for the neutral colour.

As all colours in nature, except purples, can be matched by the normal eye with some one spectrum colour (which we may call the dominant colour) more or less diluted with white light, we can, where the dominant spectrum colour of a signal is known, indicate in the terms used by a person possessing normal vision what each class of colour-blind would see.

Perhaps this is best shown as a tabulated statement:

Blue mixed with a very Neutral colour.
large proportion of
neutral colour.

Blue mixed with a
large proportion of
neutral colour.

Neutral colour.

Blue mixed with a
large proportion of
neutral colour.

Neutral colour.

The neutral colour on the Young-Helmholtz theory in the case of the red-blind, would be a peacock-green, and in that of the green-blind a purple.

The table shows that a signal exhibiting certain hues of green might be mistaken for a red one, since they both might appear to the one class green and to the other red; and that with one hue of green (differing slightly in the two cases, however) it would give the same sensation as white. In only one case, viz., that in which the dominant spectrum colour to the normal-eyed is well on the blue side of the neutral points, would the signals be distinctly different in colour.

Colours of The following table gives the wave-length in the spectrum of the railway dominant colours of the signals which have been adopted by some signal glasses. of the principal railway companies when illuminated by (1st) a light of the whiteness of the arc electric light, which does not differ much from that of day-light, and (2nd) by gas-light. The percentage of white light mixed with the spectrum colour is also shown, together with the luminosity of the light transmitted. How closely the green signals approach to the neutral points of the completely colour-blind, when the mental standard of whiteness is that of daylight, can be well judged if it be remembered that these points lie between 5,200 and 4,900 for both types (see Note c, page 304).

In a testing-room, when signal lights are used as tests, colourblind persons may possibly be able, with practice, to name the different coloured signals correctly, recognizing them by their

It

relative brightness, and by their dilution with neutral colour.
Thus, a bluish-green signal might be distinctly known by
its blue hue, whilst if yellowish-green, it might be recognized
by the neutral colour being slightly tinged with the only other
spectrum colour which they see. Again, a green whose hue,
whether pure or diluted with white, accurately coincides with
that part of the spectrum where the neutral band is situated,
might probably be mistaken for white, though, even from that,
it might be distinguished by its lower luminosity. The practical
tests the Committee have carried out confirm this view; men
who are absolutely colour-blind having passed such a test
without being detected. It might be supposed that if the
colours of signals could be rightly recognized in the testing-
room they would be equally well recognized elsewhere.
must, however, be recollected that the atmospheric conditions of
the testing-room are often very different from those which are
found outside. As a rule any judgment of the colour of a signal
which depended upon its brightness would be fallacious. A
dirty glass, or a misty atmosphere, would introduce a liability to
error. The red signal of danger might then be mistaken for
the green or white signal of safety, and vice versa. It must
also be remembered that a signal light, as a rule, has no
white light adjacent to it with which to compare it, and thus a
decision as to whether a light is neutral, or slightly coloured,
has to be arrived at under great disadvantages. We shall
presently call attention to the conditions which regulate the
choice of the colours to be used as signals; here it is sufficient to
say that, even if a green were used, whose dominant spectrum
colour lay on the blue side of the neutral bands, mistakes might
still occur, more particularly in certain conditions of foggy
weather, when white light in its passage is deprived of the blue
rays in greater proportion than the green, and the green in
greater proportion than the red (see Note d, page 305).

colour

blindness.

We have so far confined our attention to colour-blind vision Description of of the dichromatic type. Incomplete colour-blindness is less incomplete likely to lead to accident than that which is complete; but any colour-blindness, in which there is approximately a neutral or grey point in the spectrum, should be regarded with great suspicion. On the other hand, there are many people who have a slightly shortened spectrum, who are yet able to distinguish all colours, and see no neutral point. These cannot be considered to be practically colour-blind. There are again others to whom the spectrum is considerably shortened, but not to the extent that it is in complete red-blindness, and they have what is apparently a neutral point in the spectrum, lying very close to that which is found in the complete colour-blind cases. The presence of this neutral colour points to such a degree of imperfection in colour sense that it must be classed as dangerously defective. A certain and prompt recognition of a green signal colour by these last would undoubtedly be difficult under some