SCINTILLATION OF THE STARS. 111 



scurcd the ray of light and allowed it to be seen between the 

 teeth on the margin. It was supposed from the marking of a 

 counter (compteur) that the artificial light traversed 56642 

 feet, or the distance to and from the stations in Tr^Fo^ n P ar ^ 

 of a second, whence we obtain a Telocity of 191460 miles in 

 a second. 53 This result therefore approximates most closely 

 to Delambre's (which was 189173 miles) as obtained from 

 Jupiter's satellites. 



Direct observations and ingenious reflections on the ab- 

 sence of all coloration during the alternation of light in the 

 variable stars a subject to which I shall revert in the sequel 

 led Arago to the result, that, according to the undulatory 

 theory, rays of light of different colour, which consequently 

 have transverse vibrations of very different length and velocity, 

 move through space with the same rapidity. The velocity of 

 transmission and the refraction differ therefore in the interior of 

 the different bodies through which the coloured rays pass. 54 



53 Fizeau gives his result in leagues, reckoning 25 (and 

 consequently 4452 metres) to the equatorial degree. He 

 estimates the velocity of light at 70000 such leagues, or 

 about 210000 miles in the second. On the earlier experi- 

 ments of Fizeau, see Comptes rendus, torn. xxix. p. 92. In 

 Moigno, Repert. d' Optique moderne, P. iii. p. 1162, we find 

 this velocity given at 70843 leagues (of 25=1) or about 

 212529 miles, which approximates most nearly to the result 

 of Bradley, as given by Busch. 



54 " D'apres la theorie mathematique dans le systeme des 

 ondes, les rayons de differentes couleurs, les rayons dont les 

 ondulations sont inegales, doivent neanmoins se propager dans 

 T Ether avec la meme vitesse. II n'y a pas de difference a cet 

 egard entre la propagation des ondes sonores, lesquelles se 

 propagent dans 1'air avec la meme rapidite. Cette egalite de 

 propagation des ondes sonores est bien etablie experimeiitale- 

 ment par la similitude d'effet que produit une musique donnee 

 a toutes distances du lieu ou Ton 1'execute. La principale 

 difficulte, je dirai I'liniqiie difficulte, qu'on evit elevee contre le 



