MOVEMENTS IN THE STAR-DEPTHS. 59 



stars or to the imaginary circles and points on the 

 sphere which are determined by the earth's movements 

 of rotation and revolution, then, if the star be really 

 in motion, a change of place must in the long run 

 manifest itself, not indeed to ordinary vision, but to 

 the piercing scrutiny and to the yet more remarkable 

 measuring powers of the astronomical telescope. A 

 hundred years may elapse before the motion is measure- 

 able, yet the astronomer can none the less certainly 

 assure himself that the motion is taking place, since he 

 has the records of those who have gone before him, and 

 the means of satisfying himself that those records are 

 trustworthy. 



It had long been felt, however, that there was an 

 unfortunate gap in the evidence respecting stellar 

 motions. The astronomer could tell how much or how 

 little the stars were shifting on the heavens, but he 

 could obtain no measure whatever of other motions 

 which nevertheless must exist among the stars. If a 

 star were receding or approaching, no trace whatever 

 of such motion could be recognised. No instrumental 

 means could enable the astronomer to measure the 

 change of brightness due to the star's change of dis- 

 tance, since such changes must needs be infinitely small 

 compared with the actual lustre of the star. 



So that it seemed as though the astronomer must for 

 ever remain ignorant of one most important portion of 

 the stellar motions. All he could do, as it appeared, 

 was to watch the aspect of the heavens, and, as it slowly 

 changed, to infer in what way the stars were moving 



