ANALYSIS OF QUANTITATIVE PHENOMENA, 399 



for atmospheric refraction, nutation, aberration, pre- 

 cession, instrumental irregularities, &c., and can with 

 difficulty be detected among these phenomena of various 

 magnitude. But, as Galileo long ago suggested, all 

 such difficulties would be avoided by the differential 

 observation of stars, which though apparently close 

 together are really far separated on the line of sight. 

 Two such stars in close apparent proximity will be sub- 

 ject to almost exactly equal errors, so that all we 

 need do is to observe the apparent change of place of 

 the nearer star as referred to the more distant one. 

 A good telescope furnished with an accurate micrometer 

 is alone needed for the application of the method. 

 Huyghens appears to have been the first observer who 

 actually tried to employ the method practically P, but 

 it was not until 1835 that the improvement of telescopes 

 and micrometers enabled Struve to detect in this way 

 the parallax of the star a Lyrae. 



It is one of the many advantages of the observation 

 of transits of Venus for the determination of the solar 

 parallax that the refraction of the atmosphere affects 

 in an exactly equal degree the planet and the portion 

 of the sun's face over which it is passing. Thus the 

 observations are strictly of a differential nature. 



By the process of substitutive weighing it is possible 

 to ascertain the equality or inequality of two weights 

 with almost perfect freedom from error. If two weights 

 A and B be placed in the scales of the best balance 

 we cannot be sure that the equilibrium of the beam 

 indicates exact equality, because the arms of the beam 

 may be unequal or unbalanced. But if we take B out 

 and put another weight C in, and equilibrium still 

 exists, it is apparent that the same causes of erroneous 



i History of ' Physical Astronomy,' p. 549. Herschel's ' Outlines of 

 Astronomy/ 4th ed. p. 550. 



