84 COSMOS. 



quel — led Arago tc the result that, accoiimg ;o the undU' 

 iatory theory, rays of light of different color, which conse 

 quently have transverse vibrations of very different length 

 and velocity, move through space with the same rapidity. 

 The velocity of transmission and. refraction differ, therefore, 

 in the interior of the different bodies through which the col- 

 ored rays pass ;* for Arago's observations have shown that 



• " D'apres la theorie mathematique dans le systeme des ondes, lea 

 rayons de differentes coiileurs, les rayons dont les ondulations sont ine- 

 gales, doivent neanmoins se propager dans I'ether avec la m^me vi 

 tesse. II n'y a pas de diiference k cet egard entre la propagation dea 

 ondes sonores, lesquelles se propagent dans I'air avec la m^me rapidite. 

 Cette egalite de propagation des ondes sonores est bien etablie expen- 

 raeutalement par la similitude d'eftet que produit une musique doniiee 

 k toutes distances du lieu ou I'on I'execute. La principale difBculte, 

 je dirai I'unique difficulte, qu'on eAt elevee centre le systeme des ondes, 

 consistait done k expliquer, comment la vitesse de propagation des ray- 

 ons de differentes couleurs dans les corps differents pouvait etre dissem- 

 blable et servir k rendre compte de I'inegalite de refraction de ces ray- 

 ons ou de la dispersion. On a montre recemment que cette difficulte 

 n'est pas insurmontable; qu'on peut constituer I'ether dans les corps 

 inegalement denses de maniere que des rayons k ondulations dissem- 

 blables s'y propagent avec des vitesses inegales : reste a determiner, si 

 les conceptions des geometres k cet egard sont confonnes a la nature 

 des choses. Voici les amplitudes des ondulations deduites expenmen- 

 talement d'une seiie de faits relatif aux interferences : 



mm. 



Violet 0-000423 



Jaune 0-000551 



Rouge 0-000620 



La vitesse de transmission des rayons de diffirentes couleurs dans le* 

 espaces celestes est la meme dans le systeme des ondes et tout-^-faii 

 independante de I'etendue ou de la vitesse des ondulations." 



" According to the mathematical theory of a system of waves, raya 

 of different colors, having unequal undulations, must nevertheless be 

 traaismitted through ether with the same velocity. There is no differ- 

 ence in this respect from the mode of propagation of waves of sound 

 which are transmitted through the atmosphere with equal velocity. 

 This equahty of transmission in waves of sound may be well demon 

 strated experimentally by the uniformity of effect produced by music 

 at all distant^es from the source whence it emanates. The principal, I 

 may say the only objection, advanced against ti'.e undulatory theory, 

 consisted in the difficulty of explaining how the velocity of the propa- 

 gation of rays of different colors through different bodies could be dis 

 similar, while it accounted for the inequality of the refraction of the 

 mys or of their dispersion. It has been recently shown that this diffi 

 culty is not insurmountable, and that the ether may be supposed to ba 

 tirxnsmitted througli bodies of unequal density in such a manner that 

 rays of dissimilar systems of waves may be propagated through it with 

 iiiieqiial velocities; but it remains to be determined whether the views 

 advanced by geometricians on this question are in unison with the act* 

 UJ',1 nature of tliinjrs. The following; are the lengths of the und'ilat-f*^is 



