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All of these approach the earth at the rate of about eighteen miles a second; and the fifth and faintest, Delta Ursæ, though not yet measured, may be held to share their advance. One of them, moreover, Zeta Ursæ, alias Mizar, carries with it three other stars Alcor, the Arab "Rider" of the horse, visible to the naked eye, besides a telescopic and a spectroscopic attendant.
The conclusion, then, seems to be that we are not justified in assuming that the solar system is unique simply because it differs widely from the double and multiple star systems; and that we should rather regard it as probable that the vast multitude of stars which do not appear, when viewed with the telescope, or studied by spectroscopic methods, to have any attendants comparable with themselves in magnitude, have originated in a manner resembling that of the sun's origin, and may be the centers of true planetary systems like ours.
The earliest comet of first-class lustre to present itself for spectroscopic examination was that discovered by Coggia at Marseilles, April 17, 1874.
It should also be added that in 1900 spectroscopic observations by Belopolski at Pulkova gave evidence that Venus really rotates rapidly on her axis, in a period probably approximating to the twenty-four hours of the earth's rotation, thus corroborating the older conclusions.
Frank McClean's 24-inch refractor, with an object-glass prism for spectroscopic work.
But the results of spectroscopic measurements of radial velocity are independent of the distance of the star. But let us not claim too much. We cannot yet say with certainty that the stars which form the agglomerations of the Milky Way have, beyond doubt, the same average motion as the stars in other regions of the universe.
Two additional hydrogen lines, making six in all, were photographed at Tulse Hill, from the Orion nebula, in 1890; and Dr. Copeland's detection in 1886 of the yellow ray D3 gave the first hint of the presence of helium in this prodigious formation. Nor are there wanting spectroscopic indications of its physical connection with the stars visually involved in it.
But of course poetic fitness does not always rule in the land of science. SIR NORMAN LOCKYER is professor of astronomical physics and director of the solar observatory at the Royal College of Science in South Kensington. Here it is that his chief work has been done for some thirty years past. The foundation-stone of that work is spectroscopic study of the sun and stars.
These two observers at once directed their telescope armed with spectroscopic adjuncts the telespectroscope is the pleasing name of the compound instrument to the new-comer. The result was rather startling. It may be well, however, before describing it, to indicate in a few words the meaning of various kinds of spectroscopic evidence.
To make this evanescent interval perceptible, a telescope eighty feet in aperture would be required. Vogel, April 24, 1890, to belong to the novel category, with the difference, however, of possessing a nearly dark, instead of a brilliantly lustrous companion. In this case, accordingly, the tell-tale spectroscopic variations consist merely in a slight swinging to and fro of single lines.
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