Velocity of Light. 257 



traverse the immensity of space, and throws out the (Question whether 

 the stars still exist which we now see sparkle. One is astonished at 

 finding so happy a conjecture in a work whose celebrated author was 

 BO far below some of his cotemporaries in mathematical, astronomical, 

 and physical knowledge. The velocity of the reflected solar light 

 was measured by Romer (November 1675) by comparison of the 

 times of occultation of Jupiter's satellites ; and the velocity of the 

 direct light of the fixed stars by Bradley's great discovery of the 

 aberration of light (made in the autumn of 1727), — that demon- 

 stration to our senses of the earth's movement of translation in its 

 orbit; viz. of the truth of the Copernican system. In very recent 

 times a third method of measurement has been proposed by Arago, 

 by the phenomena of the light of a variable star ; for example, Algol 

 in Perseus. We have to add to these astronomical methods a ter- 

 restrial measurement, which has very recently been executed with 

 great ingenuity and success by M. Fizeau, in the neighbourhood of 

 Paris. It recalls to recollection an attempt of Galileo''s with two 

 lanterns, which did not lead to any result. 



From Romer's first observations of Jupiter's satellites, Horrebow 

 and Du Hamel estimated the time occupied in the passage of light 

 from the sun to the earth, at their mean distance apart, at 14' 7" ; 

 Carsini, at 14' 10"; Newton, which is very striking, much nearer 

 to the truth, at 7' 30". Delambre, by taking into account, among 

 the observations of his time, only those of the first satellite, found 

 8' 13" '2. Encke has very justly remarked how important it would 

 be, with the certainty of obtaining the more accordant results which 

 the present perfection of telescopes would afford, to undertake a series 

 of occultations of Jupiter's satellites, for the express purpose of de- 

 ducing the velocity of light. 



From Bradley's observations of aberration, recently discovered by 

 Rigaud of Oxford, there follows, according to the investigation of Dr 

 Busch of Kbnigsberg, for the passage of light from the sun to the 

 earth, 8' 12" -14 ; for the velocity of the light of the stars 167,976 

 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 Pulkowa, 

 it appears that the first of these numbers must be considerably in- 

 creased. The result of Struve's great investigation is 8' 17" '78; 

 whence with the aberration-constant, 20" -4451, with Encke's correc- 

 tion of the sun's parallax made in 1836, and with the value of the 

 earth's semi-diameter given by him in the Jahrbuch for 1862, we 

 have for the velocity of light 166,196 geographical miles in a second. 

 The probable error of the velocity scarcely amounts to eight geogra- 

 phical miles. Struve's result for the time which light requires to 

 reach the earth from the sun differs y^^ from that of Delambre 

 (8' 13" "2), which latter was employed by Bessel in tlie Tabulae Re- 

 giomontansB, and has been used hitherto in the Berlin Astronomical 



VOL. L. NO. C— APRIL 1851. R 



