colour instruments. In a picture or in a decorative design, gradation—that is to say, the gradual increase of the strength of colour over a given area—often occurs. In the works of the great landscape painters—such, for instance, as Turner, where the problems of atmosphere and space have interested him so greatly—gradation of colour is present almost everywhere. But it is, as it were, fixed gradation; it may begin slowly and may rapidly increase in depth and intensity over a given space until it culminates in a strong point or focus. Its rate of change may also be quite equal and steady with no rapid increase and no rapid decrease, but in both cases, once stated, it is fixed and definite, whereas when colour is produced by the colour-organ under conditions of gradation, whether it be towards intensification or of weakening of the colour, it grows, or wanes, under the eye of the spectator and can be quickened, or retarded, at the will of the executant. Gradation of colour under such conditions becomes much more interesting.
Mobile colour is also of practical aid to the
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