THE PROBLEMS OF ASTRONOMY. 87 



withstanding the success of astronomers during the present century in 

 measuring tlie parallax of a number of stars, the most recent investi- 

 gations show that there are very few, perhaps hardly more than a score, 

 of stars of which the parallax, and therefore the distance, has been 

 determined with any approach to certainty. Many parallaxes deter- 

 mined by observers about the middle of the century have had to disap- 

 pear before the powerful tests applied by measures with the heliometer; 

 others have been greatly reduced and tbe distances of tbe stars increased 

 in proportion. So far as measurement goes, we can only say of the dis- 

 tances of all the stars, except the few whose parallaxes have been 

 determined, that they are immeasurable. The radius of the earth's 

 orbit, a line more than ninety millions of miles in length, not only van- 

 ishes from sigbt before we reach the distance of the great mass of stars, 

 but becomes such a mere point that when magnified by the powerful 

 instruments of modern times the most delicate appliances fail to make 

 it measurable. Here the solar motion comes to our help. This motion, 

 by which, as I have said, we are carried unceasingly through space, is 

 made evident by a motion of most of the stars in the opposite direction, 

 just as passing through a country on a railway we see the houses on 

 the right and on the left being left behind us. It is clear enough that 

 the apparent motion will be more rapid the nearer the object. We 

 may therefore form some idea of the distance of the stars when we 

 know the amount of the motion. It is found that in the great mass of 

 stars of the sixth magnitude, the smallest visible to the naked eye, the 

 motion is about three seconds per century. As a measure thus stated 

 does not convey an accurate conception of magnitude to one not prac- 

 ticed in the subject, I would say that in the heavens, to the ordinary 

 eye, a pair of stars will appear single unless they are separated by a 

 distance of 150 or 200 seconds. Let us, then, imagine ourselves looking 

 at a star of the sixth magnitude, which is at rest while we are carried 

 past it with the motion of 6 or 8 miles per second which I have described. 

 Mark its position in the heavens as we see it to-day; then let its posi- 

 tion again be marked 5,000 years hence. A good eye will just be able 

 to perceive that there are two stars marked instead of one. The two 

 would be so close together that no distinct space between them could be 

 perceived by unaided vision. It is due to the magnifying power of the 

 telescope, enlarging such small apparent distances, that the motion has 

 been determined in so small a period as the 150 years during which 

 accurate observations of the stars have been made. 



The motion just described has been fairly well determined for what, 

 astronomically speaking, are the brighter stars; that is to say, those 

 visible to the naked eye. But how is it with the millions of faint tele- 

 scopic stars, especially those which form the cloud masses of the Milky 

 Way? The distance of these stars is undoubtedly greater, and the 

 apparent motion is therefore smaller. Accurate observations upon 

 such stars have been commenced only recently, so that we have not 



