Xll 



accepted as definitive the value of the solar parallax found by Encke 

 from his discussion of the Transits of Venus of 1761 and 1769, viz. : — 



8-57". 



Winnecke, who proposed the practical programme for the observa- 

 tions of Mars in 1862, discussed the thirteen corresponding observa- 

 tions made at Pulkowa and the Gape,* and found for the solar 

 parallax 



8-964". 



This, together with Stone's result already quoted, seemed to prove, 

 from the practical as well as from the theoretical side, that the then 

 accepted value of this fundamental constant of astronomy was 

 probably at least one thirtieth part of its amount in error. 



The question of the exact redetermination of the solar parallax 

 became at once one of supreme interest, and, as Airy put it, " the 

 noblest problem in astronomy." 



Stone threw himself into its discussion with his characteristic 

 vigour. In the 'Memoirs of the R.A.S.'f he discusses afresh the 

 meridian observations of Mars made at Greenwich in 1862, com- 

 bining them with those made at Williamstown and the Cape of Good 

 Hope, and derives for the solar parallax 



8-945". 



In May, 1865, he communicated to the R.A.S. an important 

 memoir on the " Constant of Lunar Parallax," in which he derived 

 the value of that constant, as defined by Adams, from a series of 

 observations of the moon made at the Cape Transit Circle in the 

 years 1856-61, combined with corresponding observations made at 

 Greenwich. Up to the present time Stone's result is accepted by 

 astronomers as the most reliable direct determination of this impor- 

 tant constant. 



In the ' Monthly Notices ' for April, 1867, Stone calls attention to 

 a slight numerical error in the computation of the value of the mass 

 of the moou which was employed by Le Verrier in his derivation of 

 the solar parallax from the known relations between the parallaxes 

 of the sun and moon, the mass of the moon, and the lunar equation. J 

 With this revised value of the moon's mass (computed from Le 

 Yerrier's adopted values of the constants of precession and nutation) 

 and with the value of the lunar equation (6 - 50") derived by Le 

 Verrier from his discussion of the existing meridian observations cf 

 the sun, Stone derives for the solar parallax the value 



8-91". 



* < Ast. l^aeh.,' 1409, April, 1863. 

 f Yol. 33, May, 18G4. 



X 1 Annales de 1'Observatoire de Paris,' vol. 4, p. 101. 



