CHAPTER VIII.

THE PHYSIOLOGICAL PHENOMENA OF COLOUR.

A THEORY of colour-perception, in addition to explaining the phenomena of colour-blindness, must explain the physiological phenomena of colour. The theory of psychophysical perception does this in a very complete manner, the phenomena being a necessary accompaniment of this theory.

In reading about the physiological phenomena of colour I found the most contradictory statements with regard to facts. Chevreul makes all these phenomena consistent with the red-blue-yellow theory of Brewster, and the supporters of the red-green-violet theory give the facts as corresponding in every way with this theory. It is obvious that both sets of facts cannot be true. On going through the various experiments I found that the facts as stated by Chevreul were in nearly every case correct, though his explanations are erroneous. On looking over his book it will be noticed that he was aware of facts which were inconsistent with his views. Thus, when talking about the complementaries, he, wishing to prove that, two complementaries being contrasted, the only effect is a change in intensity, says with regard to Orange and Blue, "Blue, the complementary of orange, being added to blue increases its intensity. Orange, the complementary of blue, being added to orange, increases its intensity." He then adds as a note, "Upon

repeating these observations with a deep blue and an orange which is not too red, the two colours appear commonly to become redder." I agree with the fact stated in this note. I have contrasted many blues and shades of orange, and the invariable result has been that they both appear to become redder. I have shown these pairs of colours to other persons, and their observations have agreed with mine. But what do the supporters of the Young-Helmholtz theory say with regard to these two colours? They also declare that colours differ on simultaneous contrast by the addition of the complementary. But their complementaries differ from those of Chevreul, and so the results will not be the same. Thus yellow, the complementary of blue, being added to orange makes this colour orange-yellow; whilst green-blue, the complementary of orange, being added to blue makes the latter incline to green-blue. That is to say, the predictions of each theory are opposed, and the facts agree with neither. It must be obvious to the reader that, if the facts were as stated by the supporters of the Young-Helmholtz theory -namely, that colours on being contrasted differed from each other by the addition of their complementaries they would be easily explainable by a theory of perception of difference, the explanation being that on contrast they became more unlike. But the facts are not consistent with this view, and so I had to repeat the whole of these experiments. Before discussing the various well-known phenomena of contrast, I will refer to the various sources of fallacy which I found in making my experiments.

1. Colours to be contrasted must not be placed so that reciprocal absorption can take place. Chevreul, in his excellent book, describes the different methods by means of which he obtained various greens by combining blue and yellow threads. He says that the first essential in

combining threads in this way is that the yellow and blue threads must contain little or none of the third primary colour (red), or else the brilliancy of the green will be impaired. The colour obtained in this way is produced by the absorption of the yellow and blue, and the reflection of the common factor green. In making these experiments, therefore, the contrasted substances must be placed so that reciprocal absorption cannot take place.

2. The contrasted colours should not be very small.—The reason of this is, that if the contrasting surfaces be very small an opposite effect will be produced, through the rays from both substances falling on one nerve-fibre. I made a series of experiments to prove that this was the case. I made a mosaic of small pieces of coloured cardboard, each piece being about half an inch long, and one sixteenth of an inch wide. The mosaic was made of the two colours placed alternately. The effect in every case was that the colours appeared as if the contrasting colour had been added to it; for instance, in combining blue and green in this way, the green appeared to have a shade of blue, and the blue appeared to have a shade of green. In combining blue and yellow in this way both appeared grayer, there was no tinge of green in either.

The subject may be considered under the following heads :

1. Complementary Colours.

2. After Images: (i.) Positive, (ii.) Negative.

3. Simultaneous Contrast.

4. Successive Contrast.

5. Irradiation.

6. Colour Fatigue.

1. Complementary Colours.-There are certain pairs of colours which, when mixed, give rise to a sensation of white. These colours are said to be complementary to

each other. The complementary of red is blue-green; that of orange, greenish blue; that of yellow, pure blue; and that of yellow-green, violet. The complementary of pure green is not a simple colour, but a mixture, namely rose. The complementary colours may be best examined by means of a polariscope, and thin slices of selenite of different degrees of thickness. On viewing one of these plates of selenite we shall obtain a uniform field of colour. For instance, if we examine a plate of selenite giving a field of red, on rotating the prism of the polarizer the following phenomena may be observed. The point at which the field of red is brightest having been obtained, on rotating the prism the red becomes of less and less intensity. When the prism has been rotated through 45°, the field will be white. On continuing the rotation of the polarizer the complementary colour of red, blue-green, will come into view. This bluegreen will be brightest when the prism has been rotated 90° from the starting-point. The rotation being continued, at 135° from the starting-point, the field is again white. On continuing the rotation the red again appears, and increases in intensity until the point of maximum intensity is reached at the starting-point. These colours can be shown to be exactly complementary to each other. The cause of the production of these complementary colours now remains for consideration. In accordance with. the theory of psycho-physical perception, the normalsighted will perceive any mixture of colours as a unit, a modified unit, or as white. It is, therefore, not surprising that the mixture of two colours representing the whole of the spectrum should make white. The complementary of any colour can be obtained by the subtraction of the colour itself from white light. It therefore follows that the complementary of any colour consists of a mixture of

the remaining rays of the spectrum. The position of the complementary corresponds for all persons, with a spectrum of normal length, though it is differently interpreted according to the colour-perception of the individual.

The following is a list of true spectral complementaries :

2. After-images.-After-images may be of two kindsPositive or Negative. The following will illustrate these phenomena. If we gaze very intently at an object of a red colour, and then look at a sheet of white or gray paper, we shall see an image of the object upon the paper, but of the complementary to orange, namely green-blue. Positive after-images are produced by gazing at a bright object for a short time, and then shutting the eyes. The after-image will be of the same character and colour as the object which gave rise to the sensation. It is therefore only a continuation of the original sensation.

A point in connection with the colours of negative after-images, which does not appear to have received proper consideration, is that the colour of the after-image is not the true complementary of the colour which has been looked at. For instance, the after-images of some coloured cards were as follows: