Taking ‘colour’ in this sense to stand for specific colours, not for classes or ranges of varying hues, sets of colours, where in certain spatial proportions and in certain relations of saturation, brightness and luminosity relatively to one another, excite responses of emotion and attitude with marked individual characteristics. Colours, in fact, have harmonic relations, although the physical laws governing these relations are at present unknown, and the relations themselves only imperfectly ascertained. For every colour another can be found such that the combined response to the two will be of a recognisable kind, whose peculiarities are due probably to the compatibility with one another of the impulses set up by each. This compatibility varies in a number of ways. The result is that for every colour a set of other colours is discoverable such that the response to each of them is compatible with the response to the tonic colour in a definite way.[52] A sensitive colourist feels these compatibilities as giving to these combinations of colours a definite character, which no other combinations possess. Similarly relations of incompatibility between colours can also be felt such that their combination yields no ordered response but merely a clash and confusion of responses. Colours which just fail to be complementary are a typical example. Similarly the primary colours in combination are offensive; should this precise kind of offensiveness be part of the artist’s purpose, he will, of course, make use of them. The fact that roses, sunsets, and so forth are so often found to present harmonious combinations of colour may appear a little puzzling by this account. But the vast range of close gradations, which a rose petal, for example, presents, supplies the explanation. Out of all these the eye picks that gradation which best accords with the other colours chosen. There is usually some set of colours in some harmonious relation to one another to be selected out of the multitudinous gradations which natural objects in most lightings present; and there are evident reasons why the eye of a sensitive person should, when it can, pick out those gradations which best accord. The great range of different possible selections is, however, of importance. It explains the fact that we see such different colours for instance when gloomy and when gay, and thus how the actual selection made by an artist may reveal the kind and direction of the impulses which are active in him at the moment of selection.