SCINTILLATION OF THE STARS. 109 



exception of those of the first satellite, found 8' 13"- 2 

 Encke has very justly noticed the great importance of under- 

 taking a special course of observations on the occultations 

 of Jupiter's satellites, in order to arrive at a correct idea 

 regarding the velocity of light, now that the perfection at- 

 tained in the construction of telescopes warrants us in hoping 

 that we may obtain trustworthy results. 



Dr. Busch, 81 of Konigsberg, who based his calculations on 

 Bradley's observations of aberration, as re-discovered by Rigaud 

 of Oxford, estimated the passage of light from the sun to 

 the earth at 8' 12"- 14, the velocity of stellar light at 167976 

 miles in a second, and the constant of aberration at 20"*2116 ; 

 but it would appear, from the more recent observations on 

 aberration carried on during eighteen months by Struve with 

 the great transit instrument at Pulkowa,** that the former 



n Reduction of Bradley's observations at Kew and Wansted, 

 1836, p. 22; Schumacher's Astr. Nachr., bd. xiii. 1836, 

 no. 309 ; (compare Miscellaneous Works and Correspon- 

 dence of the Rev. James Bradley, by Prof. Rigaud, Oxford, 

 1832). On the mode adopted for explaining aberration 

 in accordance with the theory of undulatory light, see 

 Doppler in the Abhl. der Kon. bohmlschen Gesettschaft der 

 Wiss. 5te Folge. bd. iii. s. 754-765. It is a point of extreme 

 importance in the history of great astronomical discoveries, that 

 Picard, more than half a century before the actual discovery 

 and explanation by Bradley of the cause of aberration, probably 

 from 1667, had observed a periodical movement of the Polar 

 star to the extent of about 20", which could " neither be the 

 effect of parallax or of refraction, and was very regular at 

 opposite seasons of the year." (Delambre, Hist, de I Astr. 

 moderne, torn. ii. p. 616.) Picard had nearly ascertained the 

 velocity of direct light before his pupil, Romer, made known 

 that of reflected light. 



52 Sebum. Astr. Nachr., bd. xxi. 1844, no. 484; Struve, 

 Etudes d 'Astr. stellaire, pp. 103, 107 (compare Cosmos, vol. i. 

 p. 144.) The result given in the Annuaire pour 1842, p. 

 287, for the velocity of light in a second, is 308000 kilomenes, 

 or 770(0 leagues ^each of 4000 metres), which corresponds 



