Notices resfpccting New Books. 73 



reference in the chromatic observation of stars. Considering, how- 

 ever, the many difficulties that beset this inquiry, it is impossible 

 not to feel the force of Sir John Herschel's assertion, that " nothing 

 short of a separate and independent estimation of the total amount 

 of the red, the yellow, and the blue rays in the spectrum of each 

 star would suffice for the resolution of the problem of astrometry in 

 the strictness of its numerical acceptation ; and this the actual state 

 of optical science leaves us destitute of the means even of attempting 

 with the slightest prospect of success." (P. 301.) Perhaps an ap- 

 proximation by instrumental means to the spectra of the brighter 

 stars ought not to be despaired of. An instance is adduced (p. 299), 

 in which Sir David Brewster accounts for the orange colour of the 

 double star X Herculis by an analysis of its light. 



The " Story of y Virginis " is one of great interest, this being 

 perhaps the most remarkable instance in which the components of a 

 binary star have been shown, by the combination of theoretical cal- 

 culation with observation, to be acted upon by their mutual attrac- 

 tions. Herschel, Encke, Miidler, Smyth, Henderson, Hind and 

 Adams, are all astronomical names that have been enlisted in the 

 theoretical investigation of the orbit of y Virginis. But no astro- 

 nomer has so diligently observed this object as Capt. Smyth. His 

 observations extend over the twenty years commencing with 1831. 

 In the month of January 1836 he pronounced it to be round, and in 

 April and May of the same year saw it elongated. Sir John Herschel, 

 in a letter from the Cape of Good Hope under the date of Feb. 27, 

 1836, says, "y Virginis, at this time, is to all appearance a single ,^ 

 star." The observations that have been employed by the theoretical 

 calculators, reach as far back as 1718. In that year Pound assigned 

 the relative position of the two stars by allineation with a known 

 star seen with the eye directed to the sky, while the other eye was 

 looking through the telescope. In the years 1719 and 1722 Bradley 

 made like observations. This mode of observing, as Sir John Her- 

 schel has shown, requires a correction for a kind of optical equation 

 between the judgements of the two eyes. Other observations were 

 made by Mayer, in 1756 ; Herschel I., in 1781 and 1803 ; Herschel 

 II. and South, in 1822; Struve and South, in 1825; Herschel II. 

 and Struve, in 1828 and 1829; Herschel II., in 1830; and Dawes, 

 in 1830 and 1831, which brings as to the date of Capt. Smyth's 

 observations. Subsequent to these there are observations of Dawes, 

 Lord Wrottesley, Mr. J.Fletcher of Cockermouth, and Mr. J.F.Miller , 

 of Whitehaven. ..^ 



Sir John Herschel attacked the theoretical problem in an ad- |^ 

 mirable and well-known communication to the Royal Astronomical 

 Society, inserted in vol. v. of their Memoirs. He uses measures of \, 

 distance, on account of their uncertainty, only for the determination ,^,. 

 of the major axis, making the values of all the other elements de- 

 pend on measures of angular position. The method is in other re- 

 spects essentially graphical, " the aid of the eye and the hand being , 

 brought in to guide the judgement in a case where judgement , 

 only, and not calculation, can be of any avail." The first essay gave 



