The particular objects which are seen in the heavens are slurs, simple points of light, and nebukc, patches of an appearance of cloudy light. Single stars, under the telescope, very frequently become double, triple, quadruple, or oven a largo cluster ; nebuloo are in some cases found to consist entirely of stars, but many remain which either are not composed of stars, or will not show themselves as such to the power of our present telescopes. It is necessary to say, in speaking of double stars, that they have been long known to exist, and that scores of observers have been diligently employed upon them during the last century and a half.
When two stars are so close together that the naked eye shows them only as one, it is possible that the coincidence may be merely optical ; that is, that the lines of their directions may be so close as to make an apparent coincidence, such as takes place between the sun and moon in an eclipse of the former, though the real distances may be very great. Such optical coincidence is suspected in various double stars, but only a long course of observation can settle the suspicion in either way. But it is now found that many double stars are connected with each other by the law of gravitation, each revolving in an ellipse about their common centre of gravity, and showing every evidence of each being retained by the other, according to the Newtonian law of gravi tation. The following stars, 7 Leonia, e Bootis, Cilerculis, 8 Serpentis, and 7 Virginia, were made out to be revolving double stars, by W. Herschel, in 1803. He had been examining these pairs under the idea of detecting the parallax from them, and in so doing he recognised their changes of relative position. Since that time, Castor, / Craw,
70 Ophiuchi, v Corona, n Corona;, Bootis, n Cassiopeia, 8 Cygni, Bootis, e (4) and e (5) Lyrae, A Ophiuci, µ Draconis, C Aquarii, CCancri, and others, have been added to the list. The periods of revolution of several have been determined, ranging from 43 to 1200 years, and the other elements of several orbits have been established. The star n Coronae has completed a revolution since it was first observed.
The most interesting of double stars is y Virginia. When observed by Herschel in 1780, the distance between the two constituent stars amounted to 5'66. Henceforward it continued gradually to decrease, until at length, in 1836, the two stars had approached so close as to appear like one star even when observed in the best telescopes. From that time the two stars have been slowly opening out from each other, until they are now nearly 4' apart. The orbit of this double star has been computed by several astronomers, including among these Sir John Herschel, who has found the period of revolution to be 182 years.
The orbits of a considerable number of double stars have been com puted in recent years, a list of which will be found in Herschel's Outlines of Astronomy' and other similar works. The most dis tinguished observers of double stars besides the elder Herschel have been Sir John Herschel, Sir James South, the Rev. Mr. Dawes, and Admiral Smyth, in this country ; and on the continent the elder Struve, Besse!, Professor 3Iiidler, Otto Struve and Professor Secchi.
The labours of the elder Struve in this field of astronomical observa tion exceed in magnitude and importance those of any other astronomer.