ABSTRACT
VISUAL sensations have two qualities,—colour and brightness. The latter term includes black, white, and all the various shades of grey. The number of distinguishable colours is enormous: it has never been determined. But it is a fact of immediate perception that the variety of colour in nature is not altogether heterogeneous: there are very many intermediate tints between this colour and that. And if we try to divide up the whole multifarious colour field, and to separate out the colours which are clearly and definitely unlike the rest, we can reduce the list of ‘pure’ colours to very small dimensions. Red, yellow, green, blue, together with black and white, prove to be the simple and ultimate qualities which we are able, so to say, to abstract from the innumerable specifically different presentations of nature. All other distinguishable colour-tones are intermediates,—a fact which is very frequently expressed in the names given to them (purplish red, orange-yellow, yellowish green, violet-blue, etc.). But these six simple qualities, again, are not co-ordinates; they evince different degrees of variety or resemblance. We are inclined to regard green as being nearer blue than yellow is, and to look on red and yellow as pretty closely related colours, even when the intermediates blue-green and orange are not present to suggest the comparison. It might be thought that this notion of colour relationship was due to our knowledge of colour sequence in the rainbow. But children who have never observed a rainbow with any degree of attention will usually connect blue with green, and red with yellow, when required to arrange the four qualities in the order of likeness.
