second. It will thus be seen that the interpretation we put on any ray of light depends solely, as far as its hue is concerned, on the number of vibrations per second produced on the retina. Increase that number of vibrations in any way, then the hue becomes more bluish; decrease the number of vibrations per second, and the hue shows more of the ruddy tinge.
From these considerations it is apparent that the hue of a light as interpreted by the eye will undergo modification if the source from which the light radiates is moving towards us or moving from us.
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Fig. 27.—The Great Bear in 36,000 years.
In order to expound the matter simply I shall suppose a case of a rather simpler type than any which we actually find in nature. Let us suppose the existence of a star emitting light of a pure green colour corresponding to a tint near the middle of the spectrum. This star pours forth each second a certain number of vibrations appropriate to its particular colour, and if the star be at rest relatively to the eye, then, we assume, the vibrations will be received on the retina at the same intervals as those with which the star emits them. Consequently we shall perceive the
