322 THE BINARY STARS. 



which the spectroscope possesses of measuring the velocity of the 

 approach or retreat of a star towards or away from the earth in the 

 hne of sight was discovered, it had been impossible to detect any 

 stellar movements except those which were at right angles to our 

 line of sight, and these only by an elaborate series of most careful 

 measurements of a star's exact position on the face of the heavens, 

 made at intervals many years apart ; but the discovery that when 

 a star is approaching us, all the transverse lines in its spectrum 

 are shifted slightly nearer the blue end of the coloured band into 

 which the spectroscope expands the star's light than their position 

 would otherwise have been, and that if the star is retreating from 

 us all these lines are moved slighdy out of their proper places in 

 the opposite direction — namely, towards the red end, at once 

 opened an altogether new field for research. But the spectroscope 

 will do more than this : it will not only tell us at once whether a 

 star is moving towards us or running away from us, quite irrespec- 

 tive of the star's distance away ; but by measuring the amount by 

 which all the lines are moved in one direction or the other, and 

 making a comparison between this amount and the known velocity 

 of light, it is possible to declare at what speed the star is moving. 

 I do not propose to deal with the method of procedure by which 

 these wonderful results are attained, as that is a subject which has 

 already recently occupied the attention of this (the Leeds Astro- 

 nomical) Society ; I wish merely to describe some of these results. 

 We will take Mizar, the centre star of the three forming the 

 tail of the Great Bear. I have already used this star as an 

 example of stars of the second class, and until observed spectro- 

 scopically it did appear with its companion to belong rightly to 

 this class, as no revolution of the companion round Mizar was 

 discernible, and both of them, together with Alcor, the small star 

 near them, appeared to possess proper motion in the same direc- 

 tion, rendering it probable that some physical connection exists 

 between all the members of the group ; but Sir Robert Ball, in his 

 book, Jn the High Heavens, describes the remarkable results which 

 have been obtained by the application of the spectroscope to 

 Mizar, the principal star of the group, by Professor Pickering. 

 Professor Pickering, it seems, has found that Mizar, as well as 

 being a very easy and beautiful telescopic double belonging to our 



