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fulfilled? Yes; they are, in the minutest particular. Not only does this theory explain all the phenomena of colourblindness in the most complete manner, but it accounts for a number of phenomena which were previously unexplainable.

On referring to the frontispiece, it will be seen that the majority of persons see six colours in the spectrum, and that these colours appear at the points which I have predicted.

In some cases seven colours are seen, and then the seventh colour appears at the point where it should appear by theory. In the first degree of colour-blindness only five colours, or points of difference, are seen in the spectrum; in the next degree four; in the next, three; then two. Then a neutral band appears at the blue-green junction, and this increases in size in different cases until total colour-blindness is reached. Therefore, the vision of the normal-sighted being hexachromic, the vision of the colour-blind is pentachromic, tetrachromic, trichromic, or dichromic. It will be noticed that the greatest difference is to be found between the three-unit and the two-unit cases of colour-blindness, the primary colours for each being quite different. The two primary colours for the two-unit are yellow and blue, and they each represent half of the spectrum. In the case of the three-unit the three primary colours are red, green, and violet. Red combined with green forms yellow; violet combined with green forms blue, so it is evident that these colours occupy the positions which I theoretically allotted to them.

The following will give the normal-sighted reader the best idea of this theory of colour-perception and the various phenomena of colour-blindness. He knows that he can see six definite colours. Let him for the five-unit imagine that five of his adjacent colours are spread over the spectrum;

for the four-unit, four; for the three-unit, three; for the twounit, two; and for the one-unit, one. Looking at the subject in this way, he will at once comprehend how it is that the colour-blind are able to recognize colours and correctly name them. As an example of this method of looking at the subject, let us represent the two-unit by the two adjacent units, blue and green. It is obvious that a normal-sighted person could distinguish different colours, even if only two were visible to him. Then colours might be named in the following way. A dark but pure green would represent red; a very bright green, orange; the brightest possible green, yellow; a bright but not quite so pure green, yellow-green; a duller and not so pure green, pure green; a mixture of the two colours, blue-green; a blue, with a trace of green, blue; the purest possible blue, violet. It will be noticed in the following chapters that this is how a certain two-unit learned to distinguish between colours, and his friends have great difficulty in getting him to name them wrongly. Again, if we consider the three-unit as represented by the three colours, red, orange, and yellow, of the normal-sighted, we see how a mixture of red and violet gives rise in the three-unit to a sensation very similar to green, as a mixture of red and yellow with the normal-sighted gives rise to a sensation very similar to orange.

To the colour-blind, colour as a quality of objects is much less than it is to the normal-sighted, and in direct proportion to the degree of colour-blindness. For this reason the colour-blind rarely make remarks about colour, and generally say that it interests them very little. In the three-unit the three most distinct colours are red, green, and violet-that is to say, the centres of their three psycho-physical units. In the simple two-unit the colours which are best seen are yellow and blue, the centre colours

of each of the units. This is a point which all simple two-unit colour-blind agree about; they say that the yellow buttercup and the blue sky give them the strongest and most contrasted sensations of colour. The three-unit find that they have a large number of superfluous colours; their vision is trichromic, whilst that of the two-unit is dichromic. The apparently superfluous colours will be confused with those which they can see distinctly; this will be especially the case with purples, browns, and grays. Rose, made up of a mixture of red and violet, is a colour with which they find especial difficulty, and according to the proportions of red and violet will the colour be classified; thus a purple containing less red than violet will be classed with the violets, whilst a rose-red containing more red than violet will be classed according to its shade with red, orange, or yellow. A rose which consists as nearly as possible of equal parts of red and violet will often be mistaken for green, in the same way as the normal-sighted fail to distinguish an orange made up of red and yellow from a pure orange reflecting the orange rays of the spectrum. The close relation which purple, green, and gray have to each other, even for the normalsighted, is shown by the changeable silks of my Pocket Test.

Colour to a person seeing the spectrum like Fig. 9 is scarcely a quality of objects at all. The individual from whom this diagram was constructed was a case of fourunit colour-blindness for the left eye, and this condition for the right he said that to the right eye the spectrum looked as if it were nearly all gray, but with a tinge of red at one end and a tinge of blue at the other. The case is described in full further on in this book.

With regard to the colours seen by each, the most typical colour will be that corresponding to the central point of

each unit. For the sake of uniformity I have not represented this in the diagram. Thus the first unit in Fig. 6 should be represented by yellow, the second by blue-violet, because these colours correspond to the central points of each unit.

The number of approximate psycho-physical units seen by any individual in the spectrum will be the number of colours he is able to see, and under no circumstances will he be able to see more than this number. We also know that if any two colours belonging to adjacent units be mixed-for instance, blue and green-we obtain a mixture which is not a fresh colour, but one possessing the characteristics of both of its components,—that is, a modified unit. Also in many cases, if we mix the colours belonging to two units, not being adjacent ones, we shall obtain a colour corresponding to that lying between the two. From these facts we can deduce definite laws of colour-perception which are applicable to all cases.

1. An individual can have no conception of a colour which does not form one of his psycho-physical colour units, or a very apparent modification of one of those units.

2. If the colours belonging to two adjacent units be mixed, an impression of both units is obtained which is plainly perceived as a mixture.

3. If two colours, not being adjacent units, be mixed, the colour between the two will tend to be revived and brought before the mind, or white will be the result in the case of pure light, gray when there is partial absorption.

4. If any number of colours be mixed, the resulting impression will be that of a unit, a modified unit, or white.

There are other alterations in a sensation which have to be taken into consideration in individual cases, besides those described in detail above. I have mentioned the influence of the sense-organ in limiting a psycho-physical

series, but the sense-organ might directly alter the apparent colour of a body through absorption; that is, the colour will be relatively different from that of other persons. For instance, the crystalline lens has a tendency to become yellow with age: all objects to a person of this kind will appear yellower than they would to a person with a normal lens. These details will be discussed fully in the chapter on the Qualitative and Quantitative Estimation of Defects of Colour-perception.

The Effect of a Shortened Spectrum upon Psycho-physical colour-perception.-In the preceding pages I have considered the effect of a lessened perception of difference, the length of the psycho-physical series being the same in each

case.

The effect which shortening of the psycho-physical colour series from any cause would have upon psychophysical colour-perception now remains for consideration.

The first obvious effect would be, that the portion of the series which was not perceived would, for that individual, be practically non-existent. Therefore, any colour consisting only of these rays would appear black, and these rays would have to be subtracted from the composition of any colour in which they formed a component part.

The junctions of the various colours will also be slightly different from that of the corresponding class with a spectrum of normal length. It is obvious that the perceptive centre can only be cognizant of sensations which are conveyed to it, and sensations which are not conveyed to it are for all purposes of perception nonexistent. For instance, let us consider the positions of the junctions of the units in a case of three-unit psychophysical perception with shortening of the red end of the spectrum. This limited spectrum will have to be divided into three, and then it will be found that both the red

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