1884.] <m the Distances of the Fixed Stars. 93 



from the nearest star subtended a smaller angle than their instru- 

 ments could measure. Bradley felt sure that if the star y Draconis 

 were so near that its parallax amounted to 1" of arc he would have 

 detected it — that is, if the earth's orbit viewed from y Draconis 

 measured 2" in diameter (or as big as a globe 1 foot in diameter 

 would look if viewed at 40 miles distant) he would have detected it. 

 But the real distances of the stars were greater than that. 



The time at last arrived when the two great masters of modem 

 practical astronomy, Bessel and Struve, were preparing by elaborate 

 experiment and study for the researches which led to ultimate 

 success. After vain attempts to obtain conclusive results by endea- 

 vours to determine the apparent changes in the absolute direction 

 of a star at different seasons of the year, both astronomers had 

 recourse to a method which, originally proposed by Galileo in 1632, 

 was carried out first on a large scale by Sir William Herschel. I 

 shall refer in the first place to the researches of the great Kussian 

 astronomer Struve. 



Astronomers had sufficiently demonstrated that the distances of 

 the stars were very great, and it was reasonable to argue that as a 

 rule the brighter stars would be those nearest to us. If, therefore, 

 two stars are apparently near each other — the one bright, the other 

 faint — the chances are that in reality they are far apart, though 

 accidentally nearly in a line. 



If two such stars are represented by S s in Diagram I., they would 



Diagram I. 



appear near each other viewed from one side of the earth's orbit at A, 

 but not so near each other viewed from B — the opposite side of the 

 earth's orbit, the red lines obviously indicating the apparent angle 

 between the stars when they are viewed from A, and the black lines 

 the apparent angle when they are viewed from B. Struve selected 

 for the star S the bright star Vega (a Lyrae). From its brilliancy he 

 considered it probably one of our nearest neighbours amongst the 

 stars, and a faint star apparently near it seemed to afford a suitable 

 representative of the really distant star s. Struve was careful to 

 ascertain that this comparison star was not physically connected with 

 a Lyrte, and he was able to prove this from the fact that whilst 

 a Lyrae has a small annual motion relative to all neighbouring stars, 

 this motion is not shared by the faint comparison star. Struve was 



