Although to the unaided eye all the stars appear single, yet in numerous instances the application of suitable optical assistance shows that many consist in reality of two stars, placed in juxtaposition so close together that they appear as one. These are termed double stars. Only 4 of these objects were known, until Sir W. Herschel, by means of the powerful telescopes constructed by himself, discovered a large number never before suspected. He observed and catalogued, altogether, about 500, which subsequent observers, especially P. G. W. Struve, and Sir J. Herschel, have augmented to near 6000.

The following have been selected by Sir J. Herschel from Struve's catalogue, as remarkable examples of each class, well adapted for observation by amateurs, who may be disposed to try by them the efficiency of telescopes.

If two stars lie very nearly in the same line of vision, whatever may be their distances, they will form an optical double star, or one whose components are only apparently, but not really joined together. Herschel, considering it extremely unlikely that this was the case, undertook, in 1778, carefully to investigate the subject.