PORTION OF THE COSMOS. VELOCITY OF LIGHT. 73 



important it would be, with the certainty of obtaining the 

 more accordant results which the present perfection of tele- 

 scopes would afford, to undertake a series of occupations 

 of Jupiter' s satellites, for the express purpose of deducing 

 the velocity of light. 



From Bradley' s observations of aberration, recently re- 

 discovered by Rigaud of Oxford, there follows, accord- 

 ing to the investigation of Dr. Busch ( 14 ) of Konigs- 

 berg, for the passage of light from the sun to the earth, 

 8' 12"-14 ; for the velocity of the light of the stars 167976 

 geographical miles in a second ; and for the constant of 

 aberration, 20"'2116 : but, from the more recent aberration- 

 observations of Struve, made for eighteen months with the 

 large transit instrument at Pulkova,( 141 ) it appears that the 

 first of these numbers must be considerably increased. The 

 result of Struve's great investigation is 8' 17"' 78; whence, 

 with the aberration-constant, 20". 4451, with Encke's cor- 

 rection of the sun's parallax made in 1835, and with the 

 value of the earth's semi-diameter given by him in the 

 Jahrbuch for 1852, we have for the velocity of light 166196 

 geographical miles in a second. The probable error of the 

 velocity scarcely amounts to eight geographical miles. 

 Struve' s result for the time which light requires to reach 

 the earth from the sun differs --^ from that of De- 

 lambre (8' 13"'2), which latter was employed by Bessel in 

 the Tabulae Eegiomontanse, and has been used hitherto in 

 the Berlin Astronomical Almanack. The discussion of 

 this subject cannot be regarded as completely terminated ; 

 but the earlier entertained supposition, that the velocity of 

 the light of the Pole-star was less than that of its companion 

 in the ratio of 133 : 134, remains subject to great doubts. 



