SIR WILLIAM HERSCHEL was the first to notice that many stars which, to the unaided vision, seemed single, were really composed of two stars in close proximity to each other. The first question to arise in such a case would he whether the proximity is real or whether it is only apparent, arising from the two stars being in the same line from our system. This question was speedily settled by more than one consideration. If there were no real connection between any two stars, the chances would be very much against their lying so nearly in the same line from us as they are seen to do in the case of double stars. Out of 5,000 stars scattered at random over the celestial vault the chances would be against more than three or four being so close together that the naked eye could not separate them, and would be hundreds to one against any two being as close as the components of the closer double stars revealed by the telescope. The conclusion that the proximity is in nearly all cases real is also proved by the two stars generally moving together or revolving round each other.
Altogether there is no doubt that in the case of the brighter stars all that seem double in the telescope are really companions. But when we come to the thousands or millions of telescopic stars, there may be some cases in which the two stars of a pair have no real connection and are really at very different distances from us. The stars of such a pair are called 'optically double.' They have no especial interest for us and need not be further considered in the present work.
After Herschel, the first astronomer to search for double stars on a large scale was Wilhelm Struve, the celebrated astronomer of