Colours may differ from each other in three respects.

1. Difference of Hue.-The best example of colours differing in hue is afforded by the spectrum. All colours must be either units or modified units.

2. Difference of Luminosity.-No coloured object can have the luminosity of a white object reflecting practically the whole of the light impinging upon it. Therefore, if we take absolute reflection as 100, a fraction of 100 will give the relative luminosity of any body.

3. Purity. The third respect in which colours may differ from each other is purity, or freedom of the colour from admixture with white light. When I speak of a colour being mixed with white light, I have a different meaning from that which is signified by most writers on colour. By the fourth Law of Colour-Perception, if any number of colours be mixed the resulting impression will be that of a unit, a modified unit, or white. Some mixtures therefore appear white, others appear coloured. It is obvious that if we mixed these two mixtures we should get a colour diluted with white. A mixture of yellow and blue gives rise to a sensation of white, as does a mixture of blue-green and red; and yet these mixtures are quite distinct, so much so that there is not an element common to both. The action of the two whites on a photographic plate at once shows how dissimilar they really are.

Ordinary white light is made up of waves differing very considerably in their refrangibility, and has a constitution quite different from that which is made up of a mixture of yellow and blue light.

It will be seen, therefore, that a colour may appear to be mixed with white light and yet contain no true white light. An example will illustrate this. If we take a beam of sunlight and allow it to pass through a coloured substance which is perfectly opaque to the blue rays, but

transparent to the rest of the spectrum, we shall obtain a very bright yellow, apparently diluted with white light. We know that a large number of the rays have combined to form yellow; for instance, the yellow-green and orange, the pure green and red. The violet and blue-green would combine to form a blue. This blue would again combine with some of the yellow to form white, which would dilute the yellow previously formed.

It will now be as well to discuss the terms which are used by artists and many writers on colour. It must be borne in mind that we obtain nearly all our colours by absorption of one or more of the constituents of white light. The process, therefore, is one of subtraction, and not of addition.

We can obtain a series of colours of the same hue varying from the brightest and purest colour on the one hand, and to black on the other. The former series are called tints of the colour, the latter shades.

The terms used by artists are based on the assumption that red, blue, and yellow are primary colours. The socalled secondary colours are orange, green, and purple. The so-called tertiaries are obtained by mixing two of the secondary colours. They are really only simple colours mixed with gray or black. It will be found that the whole system of possible colours may be obtained with the three variables which I have given.

The majority of people see six distinct colours in the spectrum-red, orange, yellow, green, blue, and violet. They have, therefore, six units of colour. Each of these units forms the basis of a series extending from the lightest to the darkest possible colour. Many persons are under the impression that brown is a distinct colour. It is not, but a darker shade in either the yellow or orange series.

We can have a very numerous series of colours if the modified units are counted as distinct colours. We can have red gradually passing into orange; orange gradually passing into yellow; and so on, to the end of the spectrum: but it is obvious that we are not dealing with distinct colours, as we can distinguish the components of the modified unit. Thus with blue-green we can plainly see that the colour consists of green and an admixture of blue. This cannot be done with the true units of colour; thus, take the two colours which are commonly supposed to make green, yellow, and blue, and compare them with green. As representative pigments, Light Chrome, Emerald, Green, and Ultramarine may be used. The three appear entirely different colours, possessing no common factor.

The spectrum gives us the purest representation of each of the six colours, but in order that the reader may be able to form a fair idea of these colours without referring to the spectrum, I will mention a few objects representing as nearly as possible a pure unit of colour.

1. Red. There is no pure red pigment in common use, all incline either to violet or orange. The common idea of red is an orange-red like Vermilion. A very fair red may be obtained by mixing Crimson Lake with Vermilion. Deep ruby glass is a very pure red. Claret is of a deep but pure red colour. The red-browns represent the deeper shades of red.

2. Orange.-Orange-peel is a very fair representative of a medium shade of this colour. Red lead is orange with a slight admixture of red. Saffron is also a very fair orange. It must be borne in mind that the deeper shades of orange do not incline to red, but form orange-brown. Copper represents a deep orange. The glow of a coal-fire is a very fair orange.

3. Yellow. The pigment Pale Chrome is an excellent

example of yellow. Sulphur is also a good yellow. Deep yellow does not incline to red, but to yellow-brown. Tan is a good example of deep yellow.

4. Green. The pigment Emerald Green is nearly a pure green. It inclines slightly to a yellow-green. Grass is a yellow-green.

5. Blue.-Ultramarine made from lapis-lazuli is the best example we have of a pure blue. A peacock's neck towards sunset is a very pure blue. This blue as daylight wanes becomes tinged with violet. full daylight is greenish blue.

A peacock's neck in

6. Violet. One of the best examples of a pure violet is the colour of the flower of some varieties of Lobelia, which is used so much in carpet bedding. The cornflower is also an example of a pure violet. Contrast a cornflower with a poppy, and the vivid contrast which red and violet form is apparent. The red colour of the poppy is heightened, and the violet colour of the cornflower also becomes of a more typical violet. The fact that red and violet form the greatest contrast of any two colours, becomes apparent in a marked degree. The best time to see a pure violet is towards sunset. At this time the violet rays are plainly visible, whilst the red rays are only imperfectly perceived. A very fair idea of a pure violet may be obtained by looking at a piece of lightblue glass, contrasted with a piece of intensely bright red glass. Simultaneous contrast prevents the red from being visible in the blue, and throws the latter into the violet. It also gives the violet the cold, chilly appearance of true violet.

CHAPTER VII.

THE COMPOSITION AND COMBINATION OF COLOURS.

IN the previous chapters I have shown that it is impossible for a normal-sighted six-unit person to see more than six colours. If only pure colours-that is, colours reflecting the corresponding rays of the spectrum, had to be dealt with, the subject would present comparatively little difficulty. But very few colours in nature are pure; thus, a piece of blue-green glass, when examined with the spectroscope, is found to be transparent to the violet, blue, and green rays.

It is unnecessary to do more than allude to the results which have been obtained by such crude experimental methods as mixing pigments. These methods are useless, because we do not know what is the exact composition of the component colours.

With regard to the combination of spectral colours. This is a great improvement on mixing pigmentary colours, because we know the exact composition of each component. There is, however, a source of fallacy which I do not think has been pointed out, and that is the possible alteration in the colour by the surface from which it is reflected. The surface from which the colour is reflected might partially absorb one of the component colours. The least fallacious method of arriving at the composition of a compound colour is to examine the light which