CHAPTER VI.

NORMAL COLOUR-PERCEPTION.

THROUGHOUT this book I use the term "normal" to signify the colour-perception of the large majority of persons, namely the six-unit. Fig. 2 of the frontispiece represents the spectrum as seen by this class of persons. The proportion which the six-unit bears to the other classes is about twenty in twenty-five. About one fifth of the number of educated persons of the male sex whom I have examined have had a diminished colour-perception. The class of the seven-unit is rare, not more than one person in several thousand seeing seven colours in the spectrum.

Since the time that Newton described the principal colours of the spectrum as being red, orange, yellow, green, blue, indigo, and violet, most writers have adhered to the division. Latterly, however, some writers on colour have demurred at the insertion of indigo, and especially at its insertion between blue and violet. If a six-unit were asked to make seven colours he would add greenish yellow to the list. It has also been pointed out that indigo is a green-blue and not a violet-blue, and therefore could not be placed between the blue and the violet. But under some conditions indigo appears as a violet-blue.

The six-unit is the class to which I belong, therefore a description of my own colour-perception will serve as an example. I see six definite colours in the spectrum

These colours

red, orange, yellow, green, blue, and violet. have the proportions marked out in Fig. 2. If a spectrum be looked at as a whole, it appears as if one colour gradually passed into another, in such a way as to make it difficult to mark out the exact junction of the units. However, when shutters are used to the eye-piece of the spectroscope, all but a small portion of the colour may be excluded. The junction can then be found with ease. I see no trace of the indigo of Newton in the spectrum, the blue gradually passes into the violet without any change in the nature of the blue. The junction of the blue and green of the spectrum has a peculiar appearance to me. The colour is not what is usually understood as a bluegreen, but appears as if the two colours overlapped at this point, just as if the torn edge of a piece of blue tissuepaper were made to overlap a similar edge of green tissuepaper. The edge looks attenuated, just as paper does when torn in the wet condition. I have not found any one who possessed a spectrum of greater length than myself; the majority of persons see the spectrum of the same length.

In this chapter I wish to show how colours should appear, and the standards of colour for comparison with other classes of persons.

A normal-sighted person sees six definite colours, and a little consideration will convince a normal-sighted person that this is the case. These colours are red, orange, yellow, green, blue, and violet. If we take a pure yellow and a pure blue and place them side by side, we see at once that there is no factor common to both. This fact is obvious, and it seems difficult to conceive by any stretch of the imagination that blue and yellow can possess a common factor. When, however, we come to deal with other colours, we find more difficulty in appreciating how

distinct they are from each other.

This is due to artistic training and imperfect colours. For instance, with red and orange. The popular idea of red is erroneous, because pure reds are uncommon. In the pigments in common use there is not a pure red. Vermilion is an orange-red; Crimson Lake a violet-red. If we mix the two we get a fairly pure red, and this colour bears no resemblance to that of an orange. Again, many substances which appear yellow in thin layers or in weak solutions, appear orange if a thicker layer be taken or a stronger solution used. This has a great tendency to make casual observers think that orange is really only a deep yellow. A few experiments will, however, demonstrate the cause of this. Let us take, for example, a solution of Methyl-Orange. In a dilute solution the colour appears as a very pure yellow. The transmitted light, being examined by the spectrum, is found to consist of the red, orange, yellow, green, and most of the blue-green and violet rays. No blue rays are visible. A slightly stronger solution being used the colour appears as orange-yellow, and the transmitted light, being examined with the spectroscope, is found to consist of red, orange, yellow, and the yellow half of the green rays. The remain ́ing rays of the spectrum are completely absorbed. If a still stronger solution is made, the colour appears as a pure orange. The transmitted light being examined with the spectroscope is found to consist of the red, orange, and yellow rays. If a stronger solution is used the colour appears as orange-red; if a very strong solution is used the colour appears red, only the red rays being transmitted. The red rays of the spectrum are by far the most penetrating, and, if a substance be transparent to these rays at all, the probability is that a thick layer of the substance will appear red. It is obvious that a classification of colours may be made with the spectrum for a basis; and, as a

matter of fact, the spectral colours are the purest. We therefore start with red, orange, yellow, green, blue, and violet. If we take two varieties of modified units it will be sufficient for all practical purposes, naming the preponderating colours last. The series will therefore be red, orange-red, red-orange, orange, yellow-orange, orangeyellow, yellow, green-yellow, yellow-green, green, bluegreen, green-blue, blue, violet-blue, blue-violet, purple, rose and rose-red. If we wish to make smaller divisions we can do so after the manner of the compass; thus greenblue-blue would indicate a colour midway between greenblue and blue. The modified unit lying between red and violet occupies a peculiar position, and it is necessary to have more than two divisions. The first colour is rose, made by mixing equal parts of red and violet, and complementary to pure green. Then we can have two more colours-purple, consisting of equal parts of rose and violet; and rose-red, made of equal parts of rose and red. The above classification is the one which I have adopted throughout this book.

In looking at the subject of colour from the standpoint of the normal-sighted, we must keep very closely to the theory of psycho-physical perception. We know that under ordinary circumstances six definite colours are seen. Let us consider how the spectrum looks to the normal-sighted under conditions of varied intensity. On looking at the ordinary solar spectrum we see that there are six colours having the proportions marked out in Fig. 2. If we diminish the intensity of the spectrum by using a finer slit for admitting light, we shall see that the spectrum changes in the following manner. First the orange band disappears, and the spectrum has a very similar appearance to that marked out by the five-unit. On still further diminishing the slit the blue disappears, and we have a very similar

spectrum to that of the four-unit. On still further diminishing the slit the yellow disappears, and we have a spectrum possessing the characteristics of that of the three-unit. On still further diminishing the size of the slit the violet disappears, and only a dull red and green are seen. On still further diminishing the slit a dull green is all that is seen. These observations show how the colour-perception of the normal-sighted may vary under different conditions. It will be noticed that a gradual diminution of intensity so far interferes with perception that it may reduce a normal-sighted person to a five, four, or three-unit. We cannot, however, reduce a normal-sighted person to the condition of the two-unit; that is to say, to the normal-sighted, red and green are always visible as definite colours, and under no circumstances are likely to be mistaken for each other. It will be noticed that, at a distance, modified units become indistinguishable from the ordinary units. As an example of this let us take three coloured glasses, blue-green, puregreen, and yellow-green, putting them in lanterns at some little distance apart. If we now walk backwards from these three lights we shall notice that the yellow-green and blue-green begin to get more and more allied, and at a certain distance the three become almost indistinguishable from shades of a pure green. It is always thus in looking at colours. The modified unit becomes indistinguishable from the unit which predominates in its composition. The modified units are rarely visible in fireworks seen at a distance. The colours of the stars might for all practical purposes be made of the six true units of colours. Persons may think that an exception should be made in favour of the modified unit rose; but they will find it impossible to tell between a light red and a rose when viewed at a distance.