134 



Sir J. HerschelVs address 



second. But below this, there certainly do seem to be indications in 

 the nature of a real parallax, which might at least suffice to raise 

 the sinking hopes of astronomers, and excite them to further 

 efforts. 



But the time arrived when the problem was to be attacked from a 

 quarter offering far greater advantages, and exposed to few or none 

 of those unmanageable sources of irregular error to which the deter- 

 minations of absolute places are liable. I mean by the measurement 

 of the distances of such double stars as consist of individuals so dif- 

 ferent in magnitude as to authorize a belief of their being placed at 

 very different distances from the eye ; or, as Struve expresses it, 

 optically and not physically double. This, in fact, was the original 

 notion which led to the micrometrical measurements of double stars : 

 but not only was anything like a fair trial of the method precluded 

 by the imperfections of all the micrometers in use until recently, but 

 the interesting phenomena of another kind, which began to unfold 

 themselves in the progress of those measurements, led attention off 

 altogether from this their original application, which thus lay dor- 

 mant and neglected, until the capital modern improvements, both in 

 the optical and mechanical parts of refracting telescopes, and the 

 great precision which it was found practicable, by their aid, to attain 

 in these delicate measurements, revived the idea of giving this 

 method, what it never before had, a fair trial. The principle on 

 which the determination of parallax by means of micrometrical obser- 

 vations of a double star turns, is extremely simple. If we conceive 

 two stars very nearly in a line with the eye, but of which one is 

 vastly more remote than the other, each by the effect of parallax, 

 will appear to describe annually a small ellipse about the mean 

 place as its centre. These two ellipses, however, though similar in 

 form will differ in dimension ; that described by the more remote 

 star being comparatively much smaller : consequently, the apparent 

 places being similarly situated in each, their apparent distance on 

 the line joining these apparent places will both oscillate in angular 

 position and fluctuate in length, thus giving rise to an annual rela- 

 tive alternate movement between the individuals both in position 

 and distance, which is greater the greater the difference of the 

 parallaxes. 



