Colour

Colour is a special property possessed by ether-waves, whose wave-lengths and vibration frequencies lie between certain approximately fixed limits. We are able to recognise in various ways and by other senses than that of sight ether-waves of widely different wave-lengths, but those with the property of colour, which can be readily recognized by this special sense of vision, have wave-lengths between 1/13000 and 1/25000 of a centimetre. It must not be supposed that such waves possess no other properties recognisable by our senses; they may exhibit in different degrees heating, chemical, and, perhaps, other effects. But we find that those waves with greatest intensity of colour are distinct from those with greatest intensity of heat or of chemical effect.

A pure colour, giving monochromatic light, is that produced by vibrations of one definite frequency or wave-length; combinations of such waves will give us any other colours. The following table gives the more conspicuous pure colours, with their frequencies or number of vibrations per second: -

Black is the absence of colour; white is a mixture of colours of all wave-lengths from the red to the ultra-violet. By prism or otherwise such white light may be resolved into its constituents, the properties of each of which may be investigated separately. White-heat means that a body has been raised to such a temperature that its particles are vibrating with all the varying frequencies capable of combining to form white light; the red-heat stage occurs long before, corresponding to a lower temperature and lower frequencies.

White may also be produced by mixture of two colours, such as yellow and ultramarine; such pairs of colours which combine to give white light are called complementary. The term supplementary has recently been given to colours which combine to give any definite colour. The effect of combining pigments generally depends on some common constituent that they may both possess. Thus pure ultramarine and yellow give white light, but the ordinary ultramarine and yellow pigments combine to give green. For both the pigments are impure and possess green, their combination thus giving a mixture of white and green.

Opaque substances may not reflect all the colours that fall on them, absorbing some and reflecting others. Thus, the colour of gold is due to the fact that it reflects back only the yellow. If no yellow existed in the light by which it is viewed, the gold would appear black. If we look through a transparent substance, its colour will be that which has passed through without being absorbed. If it contain foreign particles, just as the air contains dust, etc., we may see that colour which is reflected from these particles without subsequent absorption.

The structure of the organs of vision need not here be described, but in regard to the distinction of colour by the normal human eye, it is generally understood that the retina, upon which the light falls and over which is spread the nervous matter connected with the optic nerve, possesses three distinct kinds of nerve-elements. Each of these will respond most readily to colour-waves of one definite wave-length, that is to say, there are three physiologically primary colours that are most readily distinguished by the normal eye. Physicists differ as to what these colours are, but, roughly speaking, they are the red, green, and blue. Each of these taxes only one type of nerve-ending; combinations of them will affect more than one type. It is probable that, just as a tuning-fork will resound to notes not quite of the same pitch as it itself gives, so will the nerve-ends respond to some extent to colours not exactly of the frequency they most readily take up. And further, just as a tuning-fork persists in vibrating after the note has died away that induced it to vibrate, so the sensation of light lasts for a short space of time after the cause is removed. This is the phenomenon of persistence of vision.

Imperfection in the response of one type of nerve-element has the effect of rendering that colour partially or wholly invisible to the eye; if it existed alone, the eye would not be aware of any colour; if in combination, the eye could only respond to the other constituents of the compound colour. Such a defect is termed colour blindness, dichroic if one primary colour cannot be distinguished, monochroic if two cannot be. This defect obviously involves the inability to distinguish between colours that differ only in possessing in different quantity the unrecognisable primary as a constituent.