NATURAL PHILOSOPHY. 139 



have to add to these astronomical methods a terrestrial measurement, 

 which has recently been executed with great ingenuity and success by 

 M. Fizeau, in the neighborhood 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, Hor- 

 rebow 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"; 

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

 the truth, at T 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 occulta- 

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

 velocity of light. From Bradley 's observations of aberration, recently 

 discovered by Rigaud, of Oxford, there follows, according to the inves- 

 tigation of Dr. Busch, of Konigsberg, for the passage of light from 

 the sun to the earth 8' 12".64 ; for the velocity of the light of the stars 

 167,976 geographical miles in a second ; 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 increased. The result of Struve's great 

 investigation is 8' 17", which gives for the velocity of light 166,196 

 geographical miles in a second ; the probable error of this velocity 

 scarcely amounts to eight geographical miles. 



M. Fizeau has succeeded in executing a terrestrial measurement of 

 the velocity of light, by means of an ingeniously devised apparatus, in 

 which the artificial star, like the light of oxygen and hydrogen, is 

 returned to the point from whence it came by a mirror placed at the 

 distance of 8633 metres (28,324 English feet) ; a disc furnished with 

 720 teeth, which made 12.6 revolutions in a second, alternately stopped 

 the ray of light and allowed it to pass freely between the teeth of the 

 limb. From the indications of a counter, it was inferred that the arti- 

 ficial light traversed 17,266 metres (56,648 English feet), or twice the 

 distance between the stations, in one eighteen thousandth of a second 

 of time ; whence there results a velocity of 167,528 geographical 

 miles in a second. This result comes nearest to that of Delambre, 

 derived from Jupiter's satellites, which is 167,976 geographical miles. 

 Direct observations, and ingenious considerations on the absence of 

 any alteration of color during the change of light of variable stars, 

 have led Arago to the conclusion that rays of light which have differ- 

 ent colors, and therefore very different lengths and rapidities of trans- 

 verse vibration, move through space with equal velocities ; but that in 

 the interior of the different bodies through which the colored rays pass, 

 their rates of propagation and their refractions are different. Arago's 

 observations have shown, that in the prism the refraction is not altered 

 by the relation which the velocity of light bears to that of the earth's 

 motion. All the measurements accord in the result, that the light 6f 



* Annual Scientific Discovery, 1S50, Vol. I., pp. 145147. 



