82 COSMOS. 



approximated very remarkably to the truth when he gave 

 it at 7' 30". Delambre,^ who did not take into account any 

 of the obserA'-ations made in his own time, with the excep- 

 tion of those of the first satellite, found 8' 13"'2. Encke 

 has very justly noticed the great importance of undertaking 

 a special course of observations on the occultations of Jupi- 

 ter's satellites, in order to arrive at a correct idea regarding 

 the velocity of light, now that the perfection attained in the 

 construction of telescopes warrants us in hoping that we may 

 obtain trustworthy results. 



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

 Bradley's observations of aberration, as rediscovered by E,i- 

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

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

 167,976 miles in a second, and the constant of aberration 

 at 20"-2116 ; but it would appear, from the more recent ob- 

 servations on aberration carried on during eighteen months 

 by Struve with the great transit instrument at Pulkowa,$ 

 that the former of these numbers should be considerably in- 



1699, torn. Naii., p. 435, 475; Delambre, Hist, de VAstr. Mod., torn, ii., 

 p. 751, 782 ; Du Hamel, Physica, p. 435.) 



* Delambre, Hist, de VA-ttr. Mod., torn, ii., p. 653. 



t Reduction of Bradley^ s Observations at Keio and Wansted, 1836, p. 

 22; Schumachei-'s Astr. Nackr., bd. xiii., 1836, No. 309 (compare Mis- 

 cellaneous Works and Correspondence of the Rev. James Bradley, by 

 Prof. Rigaud, Oxford, 1832). On the mode adopted for explaining ab- 

 erration in accordance with the theory of undulatory light, see Doppler, 

 in the Abhl. der Kdn. bohmischen Gesellschaft der Wiss., 5te Folge., bd. 

 iii., s. 754-765. It is a point of extreme importance in the history of 

 gi"eat astronomical discoveries, that Picard, more than half a centuiy 

 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 VAstr. Moderne, torn, ii., p. 

 616.) Picard had nearly ascertained the velocity of direct light before 

 his pupil, Romer, made known that of reflected light. 



t Schum., Astr. Nachr , bd. xxi., 1844, No. 484 ; Struve, Etudes d'Astr. 

 Stellaire, p. 103, 107 (compare Cosmos, vol. i., p. 153, 154). The re- 

 sult given in the Annnaire pour 1842, p 37, for the velocity of light 

 in a second, is 308,000 kilomenes, or 77,000 leagues (each of 4000 

 metres), which corresponds to 215,834 miles, and approximates most 

 nearly to Struve's recent result, while that obtained at the Pulkowa 

 Observatoiy is 189,746 miles. On the difference in the abeiTation of 

 the hght of the polar star and that of its companion, and on the doubts 

 recently expressed by Struve, see Madler, Astronomic, 1849, s. 393. 

 William Richardson gives as the result of the passage of light from the 

 Eun to the earth 8' 19"-28, from which we obtain a velocity of 215,392 

 milea in a second. (Mem. of the Astron. Soc, vol. iv.. Part i.. p. 68.) 



