which the bodies approached to be sufficiently augmented, the colour of the star would seem to change from green to blue, from blue to indigo, from indigo to violet; while, if the pace were still further increased, it is absolutely certain that the waves would be poured upon the retina with such rapidity that no nerves there present would be competent to deal with them, and the star would actually disappear from vision. It may, however, be remarked that the velocity required to produce such a condition as we have supposed is altogether in excess of any known velocities in the celestial movements.
_-_Figure_28.jpg)
Fig 28.—The Great Bear as it will be 100,000 years hence.
The actual changes in hue that the movements we meet with are competent to effect are much smaller than in the case given as an illustration.
On the other hand, we may consider the original green star and the earth to be moving apart from each other. The effect of this is that the number of waves poured into the eye is lessened, and accordingly the brain interprets this to imply that the hue of the star has shifted from the green to the red end of the spectrum. If the speed with which the bodies increase their distance be sufficiently large, the green may transform into a yellow, the yellow
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