Colour is, of course, excited by the length of the waves and their frequency, red being the longest and slowest, and they diminish in length and increase in frequency in the order of the spectrum through orange, yellow, green, blue, indigo, to the shortest waves, which produce the effect of violet, the whole combined forming white. Now Hering has shown that there are only four primary colour sensations, though he at one time included black and white, thus making six. The four are red, yellow, green, and blue, which are reduced by him to two complementary colours, red and green, and yellow and blue. In our present state of knowledge the Young-Helmholtz theory of three primary colour sensations for red, green, and blue seems preferable as a working hypothesis, though it seems incompatible with anatomical and physiological facts.
Difference of colour.
All objective differences between colours, according to Helmholtz, may be reduced to differences of tone, difference of fulness (saturation), and difference of brightness. These are the three colour constants.
By tone, or hue, he means in fact difference of colour as in the spectral colours. He here refers to the vibration on a tonic scale. Fulness or purity is greatest in the pure tints of the spectrum, and becomes less in proportion as they are mixed with white light. All compound colours are less full than the simple hues of the spectrum.
Brightness or luminosity is strength of light, or amount of illumination. It is measured by the total amount of light reflected to the eye.
In nature black and white must be included among the primary colours when quality is spoken of, as light acts on black and white.
All differences of tone, therefore, are the result of combinations in different proportions of the four primary colours.
Among the defects of the eye in seeing colour, Helmholtz says, “All are red blind at the innermost portion of the field of vision, all red colours appear darker when viewed indirectly.”
The furthest limit of visible field is a narrow zone, in which all distribution of colour ceases, and there only remain differences of brightness. Probably those nervous fibres which convey impressions of green light are alone present in this part of the retina. The yellow spot makes all blue light appear somewhat darker in the centre of the field.
All these inequalities are known and more or less rectified by constant movement. As the eye becomes fatigued by bright light, so that it cannot at first answer to delicate stimulus, so it can become partially fatigued for certain colours.