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perception of sounds. The first class of the colour-blind is repre- sented by those who are unable to hear very high or very low notes. The second class is represented by those who possess what is commonly called a defective musical ear. Colour-blind individuals belonging to this class can be arranged in a series. At one end of this series are the normal-sighted, and at the other end the totally colour-blind. In the third class of the colour- blind there is defective perception of colour through the fovea or central region of the retina not being normal.

Abnormalities and defects of light-perception may be sub- divided as follows :

1. Increase or diminution in the visible range of the spectrum.

2. Defective sensibility for certain wave-lengths.

3. Increased sensitiveness for certain wave-lengths.

4. Variations in the maximum of the luminosity curve.

5. Increase or defects in the power of dark adaptation (a) Very rapid or slow adaptation; (b) very complete or imperfect dark adap- tation.

If a number of persons be examined with a bright spectrum as to the point when they first see light where the red commences and the point where violet terminates, it will be found that there are considerable variations in different cases.

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 con- siderable 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 missing portion of the spectrum, only the violet is visible to him, and so the pink appears a violet without a trace of red. This pink is therefore matched with a violet or blue very much darker than itself.

An examination of those belonging to the second class of the colour-blind will show that those who are only slightly defective will declare that there are only five colours in the spectrum, orange not being seen as a definite colour, but as a yellowish-red. Another set will be found who will state that there are only four definite colours, red, yellow, green, and violet. Those who are still more defective will state that there are only three colours in the spectrum, red, green, and violet. These describe the spectrum as red, red- green, green, green-violet and violet. Then there are those who state that there are only two colours in the spectrum, red and violet, with a neutral point in the green. This neutral division be- tween the red and the violet may in extreme cases be so large that only the ends of the spectrum appear coloured with a large grey re- gion between. Finally there are persons who see no colours in the spectrum, but see it as a colourless band varying in luminosity in its different parts. It will be seen therefore that we can classify the degrees of colour-perception according to the number of definite colours which are seen in the spectrum. Those who see seven colours may be called heptachromic, those who see six, hexachromic, those who see five, pentachromic, those who see four, tetrachromic, those who see three, trichromic, those who see two, dichromic, and finally the totally colour-blind.

It might at first be thought that this classification was artificial and that some of the classes saw exactly alike, but further examina- tion will show that this is not the case. Those who see six colours in the spectrum know that there are several varieties of green, but all these are associated by their green character, and are plainly com- pound and not simple colours; for instance, in yellow-green it is quite obvious that the colour is a mixture of yellow and green, and hence the term yellow-green correctly describes it. The trichromic designate yellow as red-green and this does not correctly describe yellow for the normal-sighted.

The Tests for Colour-Blindness. On account of the arrange- ment of signals by sea and land it is necessary that persons em- ployed in the marine and railway services should be able to recognize and distinguish between the standard red, green, and white lights, in the requisite conditions.

It is not only necessary to find out whether a man is able to distinguish between the red, green, and white lights, but to ascertain as well that he thoroughly understands what is meant by colour, and the individual character of red, green and white respectively. Too little attention has been paid to this in con- structing tests for colour-blindness, and those who have had much practical experience in testing for this defect, are aware of the ignorance which exists among uneducated persons with re- gard to colours. Many are under the impression that every shade of a colour is a fresh colour, and others have the most novel ideas with respect to colours. It is necessary that a sailor or engine- driver should be able to recognize a red, green, or white light by its character of redness, greenness^ or whiteness respectively; that he has definite ideas of colour and is able to reason with respect to them. All persons who are not able, through physical

defect, to have definite ideas of the standard colours and to be able to distinguish between them, must be excluded from the marine and railway services. An engine-driver or sailor has to name a coloured light when he sees it, not to match it. He has to say to himself, " This is red light, therefore there is danger," and this is practically the same as if he made the observation out loud. Therefore, from the very commencement we have colour- names introduced and it is impossible to exclude them. Making a person name a colour is an advantage, because the colour-name excludes the element of shade. If, as some persons have said in the past, testing by colour-names is useless, then the whole series of colour-names is useless. But if I say to a friend, " That tile is red," and he agrees with me, it is evident that one object the colour of which is by him classed as red, is also classed as red by me. The ordinary colour-names, red, blue, yellow, and green, form excellent bases for classification. The engine-driver is told that red is a " Danger " signal, green an " All Right " signal. Therefore it is necessary that he should know what is meant by these colours. It is on account of there being so many variations in hue that such great difficulty has been found in constructing an adequate test for colour-blindness, as it is the definite colours and not the variation of them of which we wish to know the number. The colour-blind see a distinct difference in hue, lumi- nosity and saturation. The normal-sighted could divide the green of the spectrum into yellow-green, green and blue-green, and would, in the majority of cases, be able to range all greens under these three classes. The dichromic colour-blind see two colours' only, and name colours as variations of these two differ- ing in luminosity and saturation; they recognize yellow by its superior luminosity and distinguish between red and green by the latter appearing of less saturation.

The test which should be used for the marine and railway services is a lantern in which the requisite conditions are repre- sented. A lantern of this kind is used by the Admiralty and the Board of Trade. It is obvious that a man who cannot distinguish the red, green and white lights in these lanterns will not be able to do so in actual practice, and this fact is easily proved by testing with signal lights.

Another test for colour-blindness, a card test, 1 is useful but it is not intended for the decisive testing of sailors or railway men, though it may be used as a supplementary test. It is for use when the lantern is not available, and is probably the simplest for demonstrating to the normal-sighted person defective colour- vision in a subject. The principle involved is the perception of difference between two colours presented in a special diagram of spots of irregular shape and various tones. On a ground of separate spots of one colour a letter is formed in spots of another colour. The test consists in discriminating between the colours, and hence recognizing the letters.

This test is useful for children as it is of importance that any- one who is colour-blind should know of it at the earliest time, so that he can avoid occupations in which an accurate colour- sense is necessary.

The wool test is a failure. It is now obsolete, as it allows over 50% of dangerously colour-blind persons to pass, and it will be noticed in certain reports that of those who were rejected by the wool test and who appealed, over 50% were found to be normal- sighted and had been rejected wrongly. The colour-blind people who can pass the wool test see a slight difference between the colours, but the smallness of this is shown by the card test.

Theories of Colour- Vision. The facts of colour-vision are quite inconsistent with the older theories of colour-vision, and modifica- tions of the theories made to explain particular cases at once give rise to difficulties in the explanation of other facts. All fundamental observations should be made with pure spectral light as the use of coloured wools, coloured papers and pigments gives rise to results which are different and due to the defects of the methods employed.

The trichromatic theory, which assumes that there are three fundamental sensations the mixture of which gives rise to all other colour-sensations, was based on the facts of colour-mixing. Un- doubtedly normal colour-vision is trichromatic in the sense of colour- mixing; therefore the term trichromatic theory is not a good one and has led to much confusion. The three-sensation theory would

1 Published by G. Bell & Sons, London, 1920.