exclusively upon those interesting objects known as binary stars. There are in the heavens many cases of two stars occurring quite close together. A well-known instance is presented in the star Epsilon Lyræ, where two pairs of stars are such near neighbours that it is a fair test of good vision to be able to separate them. But there are many cases in which the two stars are so close together that they cannot be seen separately without the aid of a telescope. We may take, for instance, the very celebrated double star Castor, well known as one of the Twins. Viewed by the unaided eye, the two stars look like a single star, but in a moderately good telescope it is seen that the object is really two separate stars quite close together. The question now comes as to whether the propinquity of the two stars is apparent or real. It might be explained by the supposition that the two stars are indeed close together compared with the distance by which they are separated from us; or it could be equally explained by supposing that the two stars, though really far apart, yet appear so nearly in the same line of vision that when projected on the surface of the heavens they seem close together. It cannot be doubted that in the case of many of the double stars, especially those in which the components appear tolerably distinct, the propinquity is only apparent, and arises from the two stars being near the same line of vision. But it is, also, undoubtedly true that in the case of very many of the double stars, especially among those belonging to the class which includes Castor, the two stars are really at about the same distance from us, and, therefore, as compared with that distance, they are really close together.
Among the splendid achievements of Sir W. Herschel,
