CHAPTER XII.

COLOUR-BLINDNESS DUE TO DIMINUTION IN THE VISUAL RANGE.

THERE is another variety of colour defect which is quite separate from diminution in the number of the psychophysical units, and that is due to shortening of the spectrum. The spectrum may be shortened for bright light, or for light of diminished intensity. The amount of shortening is very variable, and many cases of twounit colour-blindness are met with in which there is no shortening. When a spectrum of diminished intensity is shortened, it will be found that the diminution of intensity will not wholly account for the defect. For instance, let us take a colour-blind person with a spectrum shortened at the violet end for light of low intensity. If the shortened portion be cut off with a shutter, and a portion of violet internal to this be taken, it will be found that he will recognize this, even if its intensity be reduced to a point at which it is only just perceptible to the normal-sighted, and considerably below the intensity of the defective portion at the point of non-recognition. I have not been able to find a colour-blind person who failed to recognize the green as some colour or light, even when reduced to the lowest degree of intensity visible to the normal-sighted. This shortening of the spectrum, either at the red or the

violet end, appears to be very common, and is a frequent accompaniment of true colour-blindness. This condition is probably due to some defect in the retina, whereby it is unable to respond to rays of light of very low or very high refrangibility, or in some cases to both, the bright gas spectrum being shortened at both ends.

The shortened portion is indistinguishable from black; that is to say, not only is the red or violet not perceived as a colour, but it is not perceived at all. The importance of ascertaining whether there is shortening of the red end of the spectrum will be seen, when it is borne in mind that the red rays, especially those at the extreme left of the spectrum, are the most penetrating and visible when the others are obscured. There must be few who have not noticed the red appearance of the sun on a foggy day. The power these red rays possess of passing through semiopaque bodies may be demonstrated by taking about eight pieces of neutral smoked glass, and looking at the gas flame through them. On looking through a single piece of glass the gas flame appears dimmed, and not quite so yellow. On looking through three pieces, the flame is distinctly reddish, and the purity of the red is only increased by adding another piece. This red is only slightly dimmed by using all eight pieces. An examination with the spectroscope in the same manner, shows that the colour is due to a band of red on the extreme left of the spectrum. The practical outcome of this is that a colour-blind person whose spectrum is shortened is not able to see a red light under conditions in which it is very obvious to the normalsighted.

It now remains to consider the influence of a shortened spectrum upon colour-vision. The first evident fact is, that bodies reflecting only light, the rays of which occupy the shortened portion of the spectrum, appear black.

Nearly all colours are compound; that is to say, the coloured body reflects other rays than those of the colour seen. Thus a blue-green glass may transmit the green, blue, and violet rays of the spectrum. The colour of a body is usually the centre unit of all the rays reflected or transmitted. Let us suppose that we have a substance reflecting the green, blue, and three-quarters of the violet, the colour of the body being green. Then, if we had another substance which reflected the whole of the violet, it would appear blue. But with a person who could not perceive the terminal fourth of the violet, the colour would look exactly the same as the green one, and, as he could not distinguish between the two, he would be in continual difficulty with blues and greens.

All colours reflecting rays occupying the shortened portion, appear darker than they do to the normal-sighted, and are always matched with darker colours belonging to a point more internal. Thus a two-unit colour-blind with a shortened red end of the spectrum matches a red with a darker green.

It will be noticed that a shortened spectrum, especially if one end only be affected, may interfere very little with the general appreciation of shade.

If for instance, we take a case in which the red end of the spectrum is shortened, so that only three-quarters of the red of the normal-sighted is seen; then all bodies which equally reflect or transmit these rays can correctly compared, because a similar portion of light has been removed from each.

It is only when one colour reflects or transmits the rays occupying the shortened portion, and the other does not, that there is any definite interference with the appreciation of shade. Again, if neither colour reflects or transmits rays occupying the shortened portion of the

spectrum, there will obviously be no interference with the appreciation of shade.

A very common mistake due to shortening of the red end of the spectrum is the confusion of pink and blue. If a person with considerable shortening of the red end of the spectrum is shown a pink which is made up of a mixture of red and violet, the red consisting of rays occupying the extreme left of the spectrum, only the violet is visible to him, and so the pink appears as violet without a trace of red. This pink is therefore matched with a violet or blue very much darker than itself.

Mistakes which are due to shortening of the spectrum may be remedied if we subtract the rays occupying the shortened portion from the colour of confusion.

For instance, if we take a blue and a pink which have been put together as identical by a person with a shortened red end of the spectrum, and look at them through a glass which is opaque to the red but transparent to the remaining rays of the spectrum, both will appear alike in hue and shade.

A person with considerable shortening of the red end of the spectrum will look at a red light (which is so dazzlingly bright to a normal-sighted person as to make his eyes ache after looking at it closely for a few seconds), at a distance of a few inches, and remark that there is nothing visible, and that the whole is absolutely black. It is obvious that the light must only consist of rays occupying the shortened portion of the spectrum.

CHAPTER XIII.

THE CLASSIFICATION OF THE COLOUR-BLIND.

IN the preceding chapters I have discussed the causes of colour-blindness. The chief causes are―

1. Diminution in the number of psycho-physical colour units.

2. Shortening of one or both ends of the spectrum. An examination with the spectrum gives a key to the colour-vision of any person. We can record the number of units which a person sees in the spectrum, and the degree of shortening in each case. It will then be found that persons will make mistakes in accordance with the examination with the spectrum. In the first class we may include those persons who, whilst belonging to the class of the six-unit, have a spectrum which is shortened at one or both ends. The remainder of the colour-blind may be classified according to their psycho-physical colourperception. Then, if the normal-sighted be designated hexachromic, the five-unit may be called pentachromic, the four-unit tetrachromic, the three-unit trichromic, the two-unit dichromic, and the one-unit totally colour-blind. In addition, the degree of shortening of the spectrum should be recorded, if present. In the case of the twounit, the presence or absence of a neutral band should be recorded. When present, its relative size should be given. It is obvious that a classification which is based upon

K