144 PLEASANT WA YS IN SCIENCE. 



around some central point; and for reasons which need 

 not here be considered, he was led to believe that in 

 whatever direction the centre of motion may lie, the stars 

 seen in that general direction would exhibit a community 

 of motion. Then, that he might not have to examine the 

 proper motions all over the heavens, he inquired in what 

 direction (in all probability) the centre of motion may be 

 supposed to lie. Coming to the conclusion that it must 

 lie towards Taurus, he examined the proper motions in that 

 constellation, and found a community of motion which led 

 him to regard Alcyone, the chief star of the Pleiades, as the 

 centre around which the sidereal system is moving. Had 

 he examined further he would have found more marked 

 instances of community of motion in other parts of the 

 heavens, a circumstance which would have at once com- 

 pelled him to abandon his hypothesis of a central sun in the 

 Pleiades, or at least to lay no stress on the evidence 

 ierivable from the community of motion in Taurus. 



Perhaps the most remarkable instance of star-drift is that 

 observed in the constellations Gemini and Cancer. Here 

 the stars seem to set bodily towards the neighbouring part of 

 the Milky Way. The general drift in that direction is too 

 marked, and affects too many stars, to be regarded as by 

 any possibility referable to accidental coincidence. 



It is worthy of note that if the community of star-drift 

 should be recognized (or I prefer to say, when it is recog- 

 nized), astronomers will have the means of determining the 

 relative distances of the stars of a drifting system. For 

 differences in the apparent direction and amount of motion 

 can be due but to differences of distance and position, and 

 the determination of these differences becomes merely a 

 question of perspective.* 



Before long it is likely that the theory of star-drift will 

 be subjected to a crucial test, since spectroscopic analysis 

 affords the means of determining the stellar motions of 



* Here no account is taken of the motions of the stars within the 

 system ; such motions must ordinarily be minute compared with the 

 common motion of the system 



