COLOUR (Pr., Couleur ; Ger., Farbe) A person sitting in a perfectly dark room can see neither the form nor colour of the objects around him ; but the moment light is admitted he at once sees the shape of objects and also their colours. It is obvious then that to have colour there must be light. Colour is due to the suppression or absorption of some of the constituent rays of white light (see " Spec trum"). A sheet of red glass looks red because it has suppressed or absorbed that particular region of the spectrum or those colours to which red is complementary (see " Colour, Comple mentary "), and it is the residuary colours that give the observer the impression of red. Pre cisely the same thing happens with any substance which is not transparent, as, for instance, a sheet of red paper or .5 green leaf ; the light incident on its surface penetrates to a slight depth into the substance of the paper or leaf, and there meets with a material or surface which reflects the light back to the eye, but in its passage into and out of the paper or leaf the light undergoes selective absorption, and the residuum of the incident light now appears either red or green.
White, grey, and black are not colours ; the first is the sum of all the spectral rays ; grey is all the rays reduced in intensity ; whilst black is the suppression of all light, and therefore of all colour. This can be strikingly illustrated by projecting a spectrum or a series of coloured glasses or filters on to a white, a grey, and a black surface. In the first case, all the colours are seen in their original purity and strength, whereas in the case of the grey surface, the colours are still there, but they are reduced in luminosity, that is, they appear less brilliant. With a black surface, such as good black velvet, the colours are absorbed entirely.
Colour may also be formed by the interfer ence of the light rays with one another, but this is also a suppression of some of the spectral rays. (See " Interference of Light.") It is usual, therefore, to designate colours as " body colours " and " surface or interference colours." To the former class belong all coloured pigments, and to the latter those colours seen on a diffraction grating, a thin soap bubble or a peacock's tail feathers.
It is important to differentiate between the action of mixed pigments and mixed lights, as the results are not comparable. In the former
case, mixing increases in each case the suppres sion or absorption of light with each pigment used, whereas the mixing of coloured lights adds light to light. To illustrate the first point, take three printing inks, red, yellow, and blue, such as are used in trichromatic printing, and examine the absorption spectrum of each. The letters at the top of the diagram refer to the Praunhofer lines, the colours being placed underneath ; the black portions show the assumed absorptions of the inks, whilst the white portions show the light reflected, the sum of which is severally red, yellow, and blue. Now, it is obvious that by superimposing these three spectra there is no part which is transmitted by all three, and the result is total absorption of light, or black. Taking the case of three-coloured lights, by mixing them on a screen by means of a triple lantern, just the reverse of the above effect is obtained ; for convenience, let there be taken red, yellow, and blue glasses, matching approxi mately the inks referred to above. Then, con sidering not the absorptions or black portions, but the white or transmitted portions in the figure, it will be understood that the whole spectrum is transmitted and the result is white light.
The former is called subtractive colour mixing or analysis, whilst the latter is termed additive colour synthesis. It must not be overlooked that whilst pigmentary colours have been dealt with, in the case of subtractive colour analysis, the argument applies also to super imposed transparent colour filters.
The tables on the preceding page show the difference between the two systems.
In these tables the colour resulting from a mixture of any two colours is found where the vertical and horizontal colours meet. Also, the term " whitish " means that the colour appears pale—that is, mixed with white light. It is usual to designate the main or predominant colour and precede it by the colour with which it is mixed ; for instance, there may be a full or pure green ; when mixed with blue, this becomes bluish-green ; with still more blue, a pure blue-green ; with increasing quantity of blue, it becomes greenish-blue.