4 POPULAR SCIENCE MONTHLY. 



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 instru- 

 ment. The angular distance apart of the components and the direc- 

 tion of the fainter from the brighter star were repeatedly measured 

 with all attainable precision. The fine folio volume, 'Mensurse Micro- 

 metricse,' 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 tele- 

 scopes 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 ob- 

 servers 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. Dur- 

 ing the last thirty years one of the most successful cultivators of double- 

 star astronomy has been Burnham, of Chicago. He is to-day the lead- 

 ing authority on the subject. Enthusiasm, untiring industry and won- 

 derful 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. 



