XXX11 NOTKS. 



lays down, as the basis of his tables of Jupiter, 14' 10" " pro peregramlo 

 diametri semissi." The error was therefore on the increase. (Compare 

 Horrebow, Trid-uum, p. 129; Cassini, Hypotheses et Satellites de Jupiter, in 

 the Mem. de 1'Acad. 16661699, T. viii. p. 435 and 475 ; Delambre, Hist, 

 de 1'Astr. moderne, T. ii. p. 751 and 782 ; Du Harael, Physica, p. 435.) 

 ( 139 ) p. 72. Delambre, Hist, de 1'Astr. mod. T. ii. p. 653. 

 ( 14 ) p. 73. Reduction of Bradley's Observations at Kew and Wansted, 

 1836, p. 22 ; Schumacher's Astr. Nachr. Bd. xiii. 1836, No. 309. (Com- 

 pare Miscellaneous Works and Correspondence of the. Rev. James Bradley, by 

 Professor Rigaud, Oxford, 1832.) Ou the attempts hitherto made to ex- 

 plain the aberration of light on the uudulatory theory, see Doppler, in the 

 Abhandl. der kon. bohmischen Gesellschaft der "Wiss. 5te Folge, Bd. iii. S. 

 745 765. It is a circumstance deserving of particular attention in the 

 history of great astronomical discoveries, that more than half a century before 

 Bradley's actual discovery and explanation of the cause of aberration (probably 

 from 1667), Picard remarked a periodical movement of the Pole-star of about 

 20", which " can neither be the effect of parallax or of refraction, and is very 

 regular in the opposite seasons of the year" (Delambre, Hist, de 1'Astr. 

 moderne, T. ii. p. 616). Picard was on the path which might have led to the 

 discovery of the velocity of direct light, half a century before his disciple 

 Homer made known the velocity of reflected light. 



( M1 ) p. 73. Schum. Astr. Nachr. Bd. xxi. 1844, No. 484; Struve, 

 Etudes d'Astr. stellaire, p. 103 and 107 (compare Kosmos, Bd. i. S. 160; 

 English edition, p. 144). In the Annuaire pour 1842, p. 287, the velocity 

 of light is given at 308000 kilometres, or 77000 lieues (each 4000 metres), 

 in a second : this result comes nearest to the present one of Struve. It gives 

 41507 German, or 166028 English geographical miles, in a second; that of 

 the Pulkova Observatory being 41549 German, or 166196 English geo- 

 graphical miles in a second. On the difference between the aberration of the 

 Pole-star and that of its companion, and on Struve's own recently-conceived 

 doubts, see Madler, Astronomic, 1849, S. 393. A still larger result for the 

 passage of light from the Sun to the Earth is given by "William Richardson, 

 viz. 8' 19"'28, to which belongs a velocity of 165688 geographical miles 

 (Mem. of the Astron. Soc. Vol. iv. Pt. 1, p. 68). 



( 142 ) p. 74. Fizeau gives his result in leagues, 25 to an equatorial degree, 

 70000 such leagues in a .second (168000 English geographical miles) . On 

 earlier experiments of Fizeau, see Comptes rendus, T. xxix. p. 92. In 

 Moigno, Repert. d'Optique moderne, P. iii. p. 1162, the result is given at 



