Dorpat. So thorough was his work in this field that he may fairly be regarded as the founder of a new branch of astronomy. Armed with what was, at that time (1815–35), a remarkable refracting telescope, he made a careful search of that part of the sky visible at Dorpat, with a view of discovering all the double stars within reach of his instrument. The angular distance apart of the components and the direction of the fainter from the brighter star were repeatedly measured with all attainable precision. The fine folio volume, 'Mensurse Micro-metricæ,' in which his results were published and discussed, must long hold its place as a standard work of reference on the subject.
Struve had a host of worthy successors, of whom we can name only a few. Sir John Herschel was rather a contemporary than a successor. His most notable enterprise was an expedition to the Cape of Good Hope for the purpose of exploring the southern heavens with greater telescopes that had then been taken to the southern hemisphere. Herschel,
Fig. 1. Position-angle and Distance of a Double Star.
South and Dawes, of England, were among the greatest English observers about the middle of the century. Otto Struve, son of Wilhelm, continued his father's work with zeal and success at Pulkowa. Later one of the most industrious observers was Dembowski, of Italy. During the last thirty years one of the most successful cultivators of double-star astronomy has been Burnham, of Chicago. He is to-day the leading authority on the subject. Enthusiasm, untiring industry and wonderful keenness of vision have combined to secure him this position.
The particulars which the careful observer of a double star should record are the position-angle and distance of the components and their respective magnitudes. To these Struve added their colors; but this has not generally been done.
Let P be the principal star and C the companion. Let N S be a north and south line through P, or an arc of the celestial meridian, the direction N being north and S south from the star P.
