Masterpieces of Science 



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 practised 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 six or eight miles per 

 second which I have described Mark its posi- 

 tion in the heavens as we see it today; then let 

 its position 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 dtie to the magnifying power of the 

 telescope, enlarging such small apparent dis- 

 tances, that the motion has been determined to 

 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 telescopic stars, especially those which 

 form the cloud masses of the Milky Way ? The 

 distance of these stars is undoubtedly greater, 

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