THE STELLAR UNIVERSE. 



reduced to the measurement of the small interval between the 

 individuals of double stars ; and it happened fortunately, that the 

 Fig. 6. micrometers used in astronomical instruments 



were capable of measuring these minute angles 

 with much greater relative accuracy than could 

 be attained in the observations on greater angular 

 distances. To these advantages were added the 

 absence of all possible errors arising from refrac- 

 tion, errors incidental to the graduation of instru- 

 ments, from uncertainty of levels and plumb- 

 lines, from all estimations of aberration and 

 precession ; in a word from all effects which, 

 equally affecting both the individual stars ob- 

 served, could not interfere with the results of 

 the observations, whatever they might be. 



57. These considerations raised great hopes 

 among astronomers, that the means were in their 

 hands to resolve finally the great problem of the 

 stellar parallax, and Sir William Herschel accord- 

 ingly engaged, with all his characteristic ardour 

 and sagacity, in an extensive series of observations 

 on the numerous double stars, for the original 

 discovery of which science was already so deeply 

 indebted to his labours. He had not, however, proceeded far in 

 his researches, when phenomena unfolded themselves before him, 

 indicating a discovery of a much higher order and interest than 

 that of the parallax which he sought. He found that the relative 

 position of the individuals of many of the double stars which he 

 examined were subject to a change, but that the period of this 

 change had no relation to the period of the earth's motion. It is 

 evident that whatever appearances can proceed from the earth's 

 annual motion, must be not only periodic and regular, but must 

 pass annually through the same series of phases, always showing 

 the same phase on each return of the same epoch of the sidereal 

 year. In the changes of position which Sir William Herschel 

 observed in the double stars, no such series of phases presented 

 themselves. Periods, it is true, were soon developed ; but these 

 periods were regulated by intervals which neither agreed with 

 each other nor with the earth's annual motion. 



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