Double Stars. 371 



in the neighbourhood, lying in different directions from it, and 

 whose positions might be assumed as sensibly invariable. Had 

 these measures proved identical at all times, after allowance had 

 been made for the pair's proper motion, the inference would 

 have been that all these four stars were at an equal distance 

 from the earth, or, more correctly speaking, that the difference 

 of their distances was inappreciable by this, or, in fact, any 

 mode of measurement; while, on the contrary, any apparent 

 shifting of place on the part of the brighter pair with respect 

 to its minute neighbours, if it recurred at corresponding 

 seasons of the year, could only be the result of a real motion 

 in the spectator's eye, and would not merely indicate that the 

 stars in question are near enough to change their apparent 

 position when viewed from different points of the earth's orbit, 

 but give the means of estimating their distance from the 

 amount of that change. Sir W. Herschel had already attempted 

 the parallax of the stars by looking out for annual variations in 

 the apparent positions of close pairs ; but while failing in one 

 attempted discovery he, as is sometimes the case, stumbled 

 upon another of not less importance connected with the cause 

 of his failure ; in trying to find a parallax, the result of one 

 star's being widely removed in point of distance from the other, 

 he detected an orbital motion, the effect of that mutual proxi- 

 mity which rendered parallax inappreciable ; and thus his 

 original object was reserved for another generation. Many 

 years afterwards, Henderson, at the Cape of Good Hope, and 

 Bessel, at Konigsberg, undertook nearly simultaneously the 

 same important investigation ; Henderson attacking a Gentauri, 

 the most splendid double star in the whole heavens, but lying 

 too far S. to be visible in our latitudes, for the same reason 

 which determined Bessel's choice of 61 Cijgni, namely, its 

 great amount of proper motion. The priority of observation 

 is undoubtedly due to Henderson in 1882, but to Bessel belonged 

 the earlier announcement, by three weeks only, at the close of 

 1838, of the discovery of sidereal parallax. It must have been 

 an anxious time for these observers, when they were repeating, 

 night after night, and season after season, the question- 

 ings on winch depended the first step of our knowledge of the 

 dimensions of the universe ; and it must have been an hour of 



attained with a high degree of accuracy. These measurements are by no means 

 applicable peculiarly to the Sun, as the very inappropriate name would imply, but 

 are equally available for all objects in the same field. The principle of measurement 

 by double focal images was discovered by Savery, in England, and Bougixer, in 

 France, previously to the middle of the last century ; Dollond devised the great 

 improvement of halving the object-glass, and Frauenhofer constructed the first of 

 any celebrity, that mentioned in the text, which has an aperture, I believe, of 6-£ 

 inches. There is a still larger one, of 7 T % inches, by his successor Merz, at the 

 Eadcliffe Observatory, Oxford. 



