134 LECTURES 



120", and let the circles be divided into 360 parts, then one division 

 on the circle will correspond to one-third of a second between the 

 spider lines. This apparatus, called the micrometer, is placed in or 

 across the tube of the telescope near the eye piece. It is sometimes 

 furnished with a slip of metal just below the spider lines, which marks 

 the interval in seconds between them. The distance between two stars 

 or the diameter of a planet is measured by adjusting the instrument 

 so that the stars shall be bisected by the lines, or the disk of the planet 

 will lie exactly between them. The turns and divisions of the screw 

 which bring the lines together will give the angular distance sought 

 in this case to the one-third of a second. Large telescopes fitted with 

 micrometers are indispensable to researches upon the distances of the 

 fixed stars. The value of one turn of the screw is fixed by directing 

 the telescope to two well defined objects of a known angular distance 

 apart. 



Special researches upon this branch of astronomy have been made 

 by Struve, Bessel, Peters, Henderson, Maclean and Groombridge. 



In 1835 Struve, then of Dorpat, now of the Great Russian Obser- 

 vatory at Pulkowa, near St. Petersburg, commenced a series of obser- 

 vations on the bright star in the Lyre. This star has a very minute 

 companion at the distance of 43". Struve was able to detect an annual 

 variation in this distance, from which he deduced the annual parallax 

 of a Lyrse to be 0".261_, from which it follows that light would be 

 about fourteen years coming from that brilliant star to us. The light 

 which it sheds upon us to-night started on its career fourteen years ago. 

 But the researches of Bessel in connexion with the star 61 Cygni, 

 form an important epoch in the history of stellar astronomy. He 

 was furnished with a magnificent heliometer, made by Frauenhofer, 

 with a micrometer of the greatest delicacy. He brought to the re- 

 search an array of instrumental means which no previous astron- 

 omer had been able to command. His consummate skill as an observer, 

 in connexion with the exquisite instruments in his hand, enabled him 

 to furnish results which have commanded universal confidence. His 

 observations were commenced at Konigsberg in 1835, and continued 

 to 1840. His field of view is presented in figure 20, where S is the 



double star 61, a and h are two 

 minute stars, situated nearly at 

 right angles from S, the former at 

 the distance of 7', the latter at 11'; 

 the middle of the two stars S was 

 the point from which the distances 

 of a and h were measured. From 

 the configuration of the stars, if 

 S approached a, it would_, three 

 months later, approach h ; at the 

 expirationof another three months 

 it would have receded from «, and 

 then after the same interval, from 

 J). This, in fact, was the order 

 of change which was observed. 

 In other words, S revolved in a little orbit about its mean place. In 



