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CHAPTER IV.

THE PSYCHO-PHYSICAL PERCEPTION OF COLOUR.

In this chapter I propose to show what we should theoretically expect on applying the theory of psycho-physical perception to the perception of colour, with the spectrum as the physical series. This chapter, like the last, is almost entirely theoretical, the predictions being made in accordance with the theory. It will be found that the facts obtained by experiment are identical with the predictions of the theory in the minutest particular, and therefore form the strongest presumptive evidence of its truth. If the reader find difficulty in comprehending the theory as described in the foregoing chapter, let him consider that the whole theory is one of perception of difference, and work out on paper the effects of a lessened perception of difference. According to this theory, persons who are colour-blind confuse colours, not because there is any colour loss or alteration in the physical basis of colour, but because they cannot see any difference between the colours.

1. The Physical Series of Colour. It is evident that before we can have a psycho-physical series, we must have a physical series; and this is admirably represented in the case of colour by the solar spectrum. This is the most perfect example of a physical series which can be obtained. By dispersion the sun's light is spread out in the form of a series, the wave-lengths of the units gradually diminishing from the red to the violet. The presence of gaps in the solar spectrum in the shape of Fraunhofer's lines shows that the series is not a perfect one. The presence of a very large number of Fraunhofer's lines shows that the spectrum is pure, and that the rays of light do not overlap.

The points of greatest difference may be obtained by measurement of the wave-lengths. It is evident that the two points of greatest difference do not come within the visual range at all, because the waves in the infra-red present a greater physical contrast to those in the ultraviolet than any two rays in the visible spectrum. If we take the portion of the physical series represented by the spectrum, the two points of greatest physical difference are the first visible ray of red and the last visible ray of violet. The third point of greatest physical difference will be some point near the centre of the spectrum. The next two points will be found between the third point of difference and the ends of the spectrum. These and subsequent points can be found with the aid of mathematical equations.

2. The Psycho-physical Series of Colour.- When a physical series has been obtained, the mental impression of this series constitutes a psycho-physical series. The appearance of the spectrum to any person constitutes the psycho-physical colour series for that person. The question then is, What is the appearance of the spectrum to the majority of persons ? Most persons say that they can see six definite colours in the spectrum-red, orange, yellow, green, blue, and violet; and that one colour appears to shade off into those adjacent to it. We can examine the spectrum in another way-that is, by only observing a small portion at a time, either using shutters to a spectroscope, or letting the spectrum pass through a slit which only allows a small portion to pass. When the spectrum is viewed in this way it appears to be made up of a series of monochromatic bands. The size of these monochromatic bands differs with different persons; that is, a band which is monochromatic to one person is not necessarily monochromatic to another. These bands are absolutely monochromatic; that is, if a portion of green were taken, the observer could not say which was the yellow and which was the blue side of the portion of light shown. But we know that the portion of light, though apparently monochromatic, contains rays of light which differ very considerably in wave-length, therefore we have a number of physical units which cannot be distinguished from each other. These units are seen under the most favourable circumstances for the detection of any difference, the adjacent colours being excluded, and yet they appear alike. The first obvious inference to be drawn from this is, that the rays of light occupying a monochromatic band are identical for the observer as far as perception is concerned. In the psycho-physical colour series, therefore, the absolute psycho-physical colour units are portions of the spectrum which appear-when the remainder of the spectrum is shut off—monochromatic. For example, take the several varieties of green which we can distinguish. As the band of colour appears monochromatic, we could substitute an equal number of any one of the rays of light entering into its formation, without altering its appearance as far as colour is concerned. From this it is evident that nearly all the experiments which have been made by physicists with regard to colour will have to be looked at from a psycho-physical standpoint, as this is a source of error which appears to have been very generally overlooked. As we can only tell an absolute psycho-physical colour unit from the adjacent units by carefully comparing them, it is evident that the difference between them is not sufficient for practical purposes, so we come to the approximate psycho-physical colour units, that is to say, colours which can be easily distinguished from each other without comparison.

These approximate psycho-physical units, for a normalsighted person, are six in number—red, orange, yellow, green, blue, and violet.

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The above diagram represents a spectrum as seen by a normal-sighted person. A and B represent the centre points of approximate psycho-physical units, green and blue; that is, colours corresponding to these portions of the spectrum would be easily distinguished without comparison. The rays included between A and C form an absolute psycho-physical unit; that is, the portion of light included between A and C appears monochromatic. If shutters were used to the eye-piece of the spectroscope, the observer would not be able to say which was the yellow and which was the blue side of the portion of light shown. If, however, he were shown the coloured band from A to D he would be able to distinguish between the colour at A and that at D. But if he were shown a colour corresponding to D, and were asked to which portion of the spectrum it corresponded, without being allowed to compare it with other colours, he would find great difficulty in indicating the right position. There are, therefore, six definite points of difference in the spectrum, to a normalsighted observer, corresponding to the centre of each of the colours, red, orange, yellow, green, blue, and violet.

D

As each of the colours at these points must by the presence of the adjacent points blend with each other, we have the approximate psycho-physical units formed. It will be seen that if the size of the absolute psycho-physical units, as A C, be increased, the size of the approximate units will also be increased. As the spectrum remains the same length, the increase in size of the approximate units results in their re-arrangement and diminution in number, and hence colour-blindness. In the first degree of colour-blindness, five instead of six distinct points are seen in the spectrum. In the next degree four, and so on, until total colour-blindness is reached.

To return to the theoretical aspect of colour. We have to consider what are the points of difference in a psychophysical series of which the spectrum is the physical series. Let us suppose that we have a spectrum, and have to consider in what order the points of difference appear according to the number which a person is able to see. It is evident that if the perception of difference were very defective, the spectrum would appear colourless and simply brighter or darker, according to the intensity of the light; a person of this kind would be totally colour-blind, and the whole of his spectrum could be matched with varying proportions of white and black—that is, gray. If the perception of difference were not quite so defective as this, the extreme ends of the spectrum would appear feebly coloured, and the remainder gray. The spectrum would appear as nearly all gray, but with a tinge of red at one end, and a tinge of violet at the other. It will be seen that as perception improves, the tinge of red and tinge of violet will invade the gray and approach each other, because the wave-lengths need not be so proportionately different before a difference is seen. It is obvious that all the colours of the normal-sighted which are included in

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