Page:Sir William Herschel, his life and works (1881).djvu/194

172 (158 x 158 = 24,964). The light from the nearest star requires some three years to reach the earth. From a star 1,344 times farther it would require about 4,000 years, and for such a cluster as we have imagined no less than 600,000 years are needed. That is, the light by which we see such a group has not just now left it. On the contrary, it has been travelling through space for centuries and centuries since it first darted forth. It is the ancient history of such groups that we are studying now, and it was thus that declared that telescopes penetrated into time as well as into space.

Other more exact researches on the relative light of stars were made by. These were only one more attempt to obtain a scale of celestial distances, according to which some notion of the limits and of the interior dimensions of the universe could be gained. Two telescopes, exactly equal in every respect, were chosen and placed side by side. Pairs of stars which were exactly equal, were selected by means of them. By diminishing the aperture of one telescope