DOUBLE STARS and MULTIPLE STARS, stars which appear single to the naked eye, but in the telescope are re solved into two or more close together. There is no limit to the distance be tween these, but those whose components are more than 15" or 20" apart are not generally included in catalogues of double stars, and those whose distance is much less than 1" require telescopes of large aperture to separate them dis tinctly. The discovery and measurement of the position-angles and distances of the components of these objects form almost a distinct department of astro nomical observation, to which many able astronomers have given nearly all their lives. These measurements have shown that many of these doubles are true binary systems, the two components re volving round their common center of gravity in periods that range from 10 to 12 years, to unknown thousands and obeying, so far as these motions can show, the same law of gravitation that regulates the motion of the solar sys tem. Sir William Herschel was the
great pioneer in the field of discovery of these objects, but his measures were of comparatively little accuracy, and the elder Struve may be considered as the first astronomer to lay the foundation of this branch of research. The field of first discovery of these double stars has thus far been principally covered by five astronomers, and most of the known doubles are usually called by the cata logue number of the works or discovery lists of these five men, with a distinguish ing letter for each, although it is very generally the custom to call the star by the catalogue number of the elder Struve if it is contained in the "Mensurm Micrometricw." These five astronomers, with their distinguishing letters, are as follows: Herschel, Sir William Struve, F. G. W. 2 Herschel, Sir John Struve, Otto 0 E.
Burnham, S. W