THE MOVEMENTS OF THE STARS. 269 



I must, however, be careful to explain that though I have 

 thus drawn an antithesis between the spectroscopic method 

 of observing proper motions and the telescopic method, 

 yet that from another point of view the two methods 

 are to be regarded rather as complementary to each other. 

 Each purports to tell us something which the other is wholly 

 incompetent to tell. This will be plain if we endeavour 

 to realise the character that the motion of a star will 

 usually present. Let us suppose that the only movement 

 possessed by the star was directly towards the observer, or 

 directly from him, that there was, in fact, no movement 

 across the line of vision. In this case the star would never 

 appear to change its place with respect to the stars in its 

 neighbourhood ; in fact, the only conceivable alteration in 

 the telescopic appearance of the star would be, that in the 

 course of time it would become brighter if approaching or 

 fainter if receding. But any change of this kind would 

 be so insignificant that we can afford to disregard it. On 

 the other hand, it might happen that the movement of the 

 star was such as to lie entirely across the celestial sphere, 

 so that it was neither approaching to nor receding from 

 the observer. In this case the entire proper motion would 

 disclose itself by the displacement of the star relatively to 

 the other stars in the heavens, and these are precisely 

 the circumstances in which the old telescopic method is 

 available. In fact, the spectroscope will only show motion 

 directed along the line of sight, while the telescope 

 only shows the motion which is perpendicular to the line 

 of sight. Thus each of the methods exhibits that par- 

 ticular movement which the other is incompetent to 

 perceive. Of course it will not often happen that the 



