into an orange, the orange into a red; while a still greater velocity is, at all events, conceivable which would cause the undulations to be received with such slowness that the nature of the light could no longer be interpreted by any nerves that the eye contains, and from the mere fact of its rapid motion away from us the star would become invisible. Here again we must add the remark that the actual velocities animating the heavenly bodies are not large enough to allow of the extreme results now indicated.
However, in the actual circumstances of the celestial bodies it seems impossible that any change of hue recognisable by the eye could be attributed to movement in the line of sight. Nor does this merely depend on the circumstance that the velocities are too small to produce such an effect. It must be remembered that the case of a star which dispenses light of perfect simplicity of composition is one that can hardly exist among the heavenly bodies, though it may be admitted that there is a certain approach to it in one or two remarkable cases. It is, however, much more usual for the light from a star to be of a highly composite type, including rays not only from all parts of the visual spectrum, but also rays belonging to the ultra-violet region, as well as others beyond the extreme red end. The effect of the retreat of a star, so far as its colour is concerned, is that though the green is shifted a little towards the red, a bluish hue moves up to supply the place of the green, and as a similar effect takes place along the entire length of the spectrum, the total appearance is unaltered.
It is a fortunate circumstance that the lines in the spectrum afford a precise means of measuring the extent
