Looking at the multitude of double and multiple stars, the question can scarcely fail to suggest itself: Is there any real connection between the stars which thus appear so close to one another? It can be readily understood that the mere fact of their appearing close together in the field of the telescope does not necessarily imply real closeness. Two gas-lamps, for instance, may appear quite close together to an observer who is at some distance from them, when in reality they may be widely separated one from the other--the apparent closeness being due to the fact that they are almost in the same line of sight. No doubt many of the stars which appear double in the telescope are of this class--'optical doubles,' as they are called, and are in reality separated by vast distances from one another. But the great majority have not only an apparent, but also a real closeness; and in a number of cases this is proved by the fact that observation shows the stars in question to be physically connected, and to revolve around a common centre of gravity. Double stars which are thus physically connected are known as 'binaries.' The discovery of the existence of this real connection between some double stars is due, like so many of the most interesting astronomical discoveries, to Sir William Herschel. At present the number of stars known to be binary is somewhat under one thousand; but in the case of most of these, the revolution round a common centre which proves their physical connection is extremely slow, and consequently the majority of binary stars have as yet been followed only through a small portion of their orbits, and the change of position sufficient to enable a satisfactory orbit to be computed has occurred in only a small proportion of the total number. The first binary star to have its orbit computed was Xi Ursæ Majoris, whose revolution of about sixty years has been twice completed since, in 1780, Sir William Herschel discovered it to be double. The star which has the shortest period at present known is the fourth magnitude Delta Equulei, which has a fifth magnitude companion. The pair complete their revolution, according to Hussey, in 5·7 years. Kappa Pegasi comes next in speed of revolution, with a period of eleven and a half years, while the star 85 of the same constellation takes rather more than twice as long to complete its orbit. From such swiftly circling pairs as these, the periods range up to hundreds of years. Thus, for example, the well-known double star Castor, probably the most beautiful double in the northern heavens, and certainly the best object of its class for a small telescope, is held to have a period of 347 years, which, though long enough, is a considerable reduction upon the 1,000 once attributed to it.