THE HERSCHELS AND THE STAR-DEPTHS. 5 



observer might at one time notice that his telescope 

 had to be pointed in a certain direction to bear on a 

 particular star, while six months later (when the earth 

 would be 180,000,000 of miles from the spot she had 

 occupied before) the observer might try to note whether 

 his telescope required to be pointed in some slightly 

 different direction to bear on the star. But in the 

 meantime the stand of the telescope might have been 

 slightly moved, as by the sinking of a pier, or even by 

 changes due to greater warmth or cold. The air might 

 not act precisely in the same way on the rays from the 

 star. The observer's own powers might have varied, 

 or rather, these and other like changes must inevitably 

 take place to some extent, however slight ; and it had 

 begun to be known in Sir W. Herschel's time that the 

 slightest possible error of the kind would suffice to 

 render any attempts at measurement ineffective. 



Herschel at once suggested a means of overcoming 

 all these difficulties. What we want, he reasoned, is 

 to tell towards what point of the heavens a star seems 

 to lie, at different seasons, and the nearer the star the 

 more it would seem to shift. A star so far off as not to 

 be visible without a powerful telescope will not seem 

 to shift at all ; for it must probably be twenty or thirty 

 times farther away than the bright stars, and we know 

 that even these shift so slightly that we cannot be sure 

 they shift at all. What is to prevent us, then, from 

 regarding one of these faint and therefore very distant 

 stars as a sort of index-point from which to measure 

 the minute excursions of some bright star close by it 



