If a paper covered with vermilion is placed successively in various regions of the spectrum, it appears red in the red region, yellow in the transition region, and black in all the others. Its surface, whilst readily reflecting the red and spectral yellow, is not able to reflect the green, blue-green, and blue-violet ; it absorbs and, as it were, destroys these radiations. In white light the surface appears red because of the pre dominance of red in the light which it reflects. The vermilion layer will appear always red when illuminated by a light which contains red ; and black in all other cases. The colour is thus not actually in the vermilion, but in the light.
Similarly, a yellow flower appears successively red, yellow, or green in the corresponding spectral regions, and black in all the others.
No natural or manufactured object, no matter by what process its surface is covered, can even approximately be considered as monochromatic, that is to say, as reflecting only radiations of very nearly the same wave-lengths. Pure spectral colours do not exist in Nature (unless in the rain bow) ; all coloured objects, natural or artificial, extinguish more or less completely some of the radiations of the light which shines on them, and it is the net result of the reflection of the other radiations which the eye observes. Colour is therefore the result of a subtraction process. White light becomes coloured because certain of the radiations in it are extinguished. It is therefore logical to define the colours of things by the spectral regions which they absorb. A bright yellow like that of the petals of a buttercup is due to the absorption of the blue-violet region and to the free reflection of the two other regions, green and red. If, on the other hand, an object reflects to the eye only the yellow radiations, these are such a small portion of white light (less than i per cent) that the object appears almost black, or at least a very deep olive.
By studying successively different colours, selected for the utmost purity and intensity, it can be shown, at least to a first approximation, that some of them absorb simultaneously two of the chief spectral regions that we have con sidered, whilst others, absorbing only one of these regions, appear to be much more luminous. The table given below sets out some data on this point.
This table shows us the existence of a colour not having any spectral equivalent. The purest
type of this is the intense rose colour given by dyes such as rhodamine, rose Bengal, or erythrosine.
6. The colours which each reflect two of the chief regions of the spectrum are known as the primary colours by painters and printers, because by superposing them two at a time, thus adding together their respective absorptions (mixing of colours by subtraction of light), the inter mediate colours can be obtained which only reflect one chief spectral region. For example, by superposing a peacock blue, which absorbs the red, and a yellow, which absorbs the blue violet, we obtain a green, since this is the only spectral region transmitted simultaneously by the two superposed colours. The mixtures thus obtained are necessarily darker than each of the mixed colours.
By placing close to one another little dots of colour so small that when looked at by the eye from a normal distance they appear blended into a single colour (the technique of the pointillist and of Autochrome), the mixing of colours is brought about by the addition of lights. The colour of the mixed light, for the reason that it is made up of the radiations reflected by each of the colours in the mixture, is more luminous than each of the colours separately.
7. The colour of a given substance depends to a very considerable extent on its state of division and on the medium in which it is. Many cases are known in which crushing a substance to a powder decreases its coloration. For example, blue copper sulphate appears white after being powdered. In addition to the light which has penetrated into the substance and which comes out from it coloured, due to the absorption of certain of the radiations in the incident white light,' there is always a certain proportion of white light reflected or diffused from the surface without alteration in colour, and which diminishes the actual coloration of the substance. In the case of a polished surface the white light is reflected almost entirely in the direction determined by the laws of reflection from mirrors. It is thus only in this one direction that the colour is diminished by reflection, and sometimes even almost completely lost in the considerable excess of white light reflected ; in other directions the coloration appears with maximum intensity.