AN EVENING WITH THE TELESCOPE. 123 



though in the same line of sight. By a careful series of 

 measures made in the course of an entire year, there is an 

 apparent displacement of one of these stars relatively to 

 the other, caused by the fact that the earth has been ever 

 changing its position in the course of its annual revolution 

 around the sun. The method fails if the star's distance be 

 beyond a certain limit, as it actually is with the great 

 majority of the bodies to which the method has been ap- 

 plied. The difficulty of applying this process to the 

 cluster in Perseus is, that we have no clue to guide us in 

 the choice of the pair of stars which would be suitable ; 

 one of them must lie in the cluster, the other must be far 

 behind it. If we had any means of identifying a pair of 

 stars which were certainly so situated, the inquiry would 

 be certainly undertaken ; but it would not improbably 

 happen, if a pair of stars were chosen at random, that they 

 would be both in the cluster, and thus the attempt would 

 be abortive. It is doubtless this feeling which has pre- 

 sented astronomers from devoting their attention to an 

 irduous and protracted series of observations, of which 

 the result would not improbably be futile. 



