The explanation of simultaneous contrast must be considered apart from complementary colours. The phenomena are most marked when the colours contrasted correspond to near positions in the spectrum. The theory that each colour differs by the addition of the complementary of the other would not explain the differences, because the contrast would be greater in cases in which the colours were not so closely allied. To take as an illustration a pure yellow and a yellow tinged with green. The complementary of yellow, namely blue, added to the greenish yellow, makes a greenish white. The complementary of greenish yellow is a blue differing very little from a pure blue, being inclined towards violet-blue. The addition of this complementary to the pure yellow would make it white, tinged with blue. Whereas the effect of contrasting the two colours is that the yellow inclines to orange, and the greenish yellow to yellow-green.

Again, let us take yellow and green and contrast them. It is obvious that the complementaries of these two colours differ much more than those of greenish yellow and yellow. Thus the complementary of yellow, namely blue, added to green makes blue-green, whilst the complementary of green, namely rose, added to yellow makes pink. Now, yellow and greenish yellow differ much more markedly when contrasted than yellow and green do.

The phenomena of simultaneous contrast must be considered purely as measuring the differences between colours. When one colour is contrasted with another the difference between the two becomes more marked-that is to say, the lower colour apparently occupies a position a little lower down the scale, and the higher colour a position a little higher up the scale. The closer the scale the more

positions occupied by colours on the

marked the differences become. We have to consider the

effect of the modified unit purple. Does a blue contrasted with a violet make the latter appear more purple? It is obvious enough that if once an element of red be admitted in the violet the colour is no longer a true violet, but one of the modified units between violet and red. But the contrast of blue with a violet which has or has not an element of red will make that colour incline to red. This does not make the colour series a circle, but is simply due to the fact that the unit red is capable of combining with the unit violet, to give rise to the modified unit purple.

The question now comes for consideration. Are these phenomena due to changes in the retina, or to changes in the brain? I am inclined to think that the phenomena are partly retinal and partly cerebral, and the following experiments give support to this view.

1. A definite complementary, having the shape of the object looked at, may be seen even if the eyes be kept moving during the whole of the experiment, so that the coloured object shall occupy respectively every portion of the visual field. The complementary is not so vivid as that produced by a fixed gaze.

2. If we look intently at a coloured object with the left eye, the right being bandaged, and then suddenly close the left eye and look at a piece of white paper with. the right eye, a complementary will be seen. This will be of a less intense character than if seen with the left eye.

3. If we bandage the right eye and obtain a complementary with the left, on looking at this complementary for a short time, and then closing the left eye and looking at a piece of paper with the right, we shall see an afterimage having the colour of the original object.

A careful consideration of the facts given above will show that they may be classified under two heads :1. Those due to psycho-physical perception.

2. Those due to colour-fatigue of the retina and brain. The first class includes all those changes in which the colour appears to be shifted to a higher or lower point in the colour-scale. The second class includes all those changes in which a complementary colour is apparently seen. The changes in all cases are greater when both effects are combined, and least when they are opposed. As an instance of this, let us take a series of coloured cards and put a small piece of gray paper on each. It will be noticed that the greatest effect is produced on the green and rose cards, the gray paper in each case being tinted of a decided rose or green, according to the colour of the card. Again, the least effect is produced in the case of the red and violet cards. In the case of these latter, it is difficult to detect any alteration in the colour of the gray. In the case of the green and rose cards, both factors tend to make the gray appear tinted with the complementary colour.

The explanation of complementary colours by my theory of retino-cerebral perception is as follows. When the colour-perceiving centre in the brain has just been perceiving rays of a certain kind, it and the retina are directly afterwards less responsive to those rays than to rays of a different degree of refrangibility, and so surrounding objects will appear to be deficient in that colour. For instance, if a person has been looking at a red object and then looks at a piece of white paper, the perceptive centre not being actively responsive to the red rays, the white appears to be deficient in the red, and is perceived as blue-green, which is the colour obtained by mixing the remaining rays of the spectrum. As I have previously stated, the blue-green seen will not be the true complementary of red, but a much bluer colour, really a greenish blue. This is due to contrast.

CHAPTER IX.

OBJECTIONS TO PREVIOUS THEORIES OF COLOUR-PERCEPTION.

A CONSIDERATION of the facts which I have given, shows that neither the Young-Helmholtz nor Hering's theory of colour-perception will explain the phenomena of colourblindness in a satisfactory manner.

With regard to the former of these theories, Helmholtz modifying Young's theory of colour-perception, assumes that there are in the retina three sets of nerve-fibres, the stimulation of the first set giving rise to the sensation of red, of the second the sensation of green, and the third the sensation of violet. He assumes that all kinds of light stimulate all three sorts of fibres, but in a different proportion. The following diagram shows the proportional stimulation of each set of nerve-fibres by different varieties of pure light.

Fig. 12.-Diagram of the three primary colour-sensations. (Holmgren.)

No. 1 curve corresponds to the red set of fibres, No. 2 to the green set, and No. 3 to the violet set. The height of the curve represents its proportional stimulation by

the variety of light indicated by the letter. Thus pure yellow light stimulates the red and green sets of fibres considerably, and the violet set slightly, the resulting sensation being yellow.

Holmgren has applied this theory to the phenomena of colour-blindness. He divides the colour-blind into the following classes :

A. Total colour-blindness, in which only one set of fibres is present in the retina, and so the individual can only perceive colours as varying degrees of light and shade. B. Partial colour-blindness, in which one set of fibres is completely absent. There may be three varieties :— (i.) Red-blindness. (ii.) Green-blindness. (iii.) Violet-blindness.

C. Incomplete colour-blindness, in which one or more of the three sets of fibres are defective.

The following objections may be urged against this theory :

1. Perception of Shade.-First with regard to those cases which have been classed under the heads of complete red-blindness, complete green-blindness, and complete violet-blindness.

If one set of fibres, responding variably to rays of light of different degrees of refrangibility, were removed, it ought to follow that the perception of light and shade of a colour-blind person should differ from the normal in a corresponding degree. Thus we can represent either of the above varieties of colour-blindness by the preceding diagram, with the corresponding curve removed. In the case of red-blindness we should remove the red curve, green-blindness the green curve, and violet-blindness the violet curve. This, according to Holmgren, gives us a key to the colour-perception of the colour-blind.