That is, a medium like red glass, for instance, which
would cut off the blue light more than the red light. Such a medium
would render the blue end of the spectrum of a distant star much
fainter, as compared with the red end, than in the case of a near star.
A measure of the relative intensity of the two rays would servo to
measure the distance, or thickness of the absorbing medium. The effect
would be the same for all stars of the same class of spectrum. It could
be tested by the stars forming a cluster, like the Pleiades, which are
doubtless all at nearly the same distance from us. The spectra of stars
of the tenth magnitude, or fainter, can be photographed well enough to
be measured in this way, so that the relative distances of nearly a
million stars could be thus determined.
Another method which would have a more limited application, would depend
on the velocity of light. It has been maintained that the velocity of
light in space is not the same for different colors. Certain stars,
called Algol stars, vary in light at regular intervals when partially
eclipsed by the interposition of a large dark satellite.
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