84 COSMOS. 



quel led Arago to the result that, according to the undu- 

 latory 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, les 

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

 gales, doivent neanmoins se propager dans I'ether avec la meme vi- 

 tesse. H n'y a pas de difference a cet egard entre la propagation des 

 ondes sonores, lesquelles se propagent dans 1'air avec la memo rapidite. 

 Cette 6galit6 de propagation des ondes sonores est bien etablio experi- 

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

 & toutes distances du lieu ou 1'on 1'execute. La principale difficulte, 

 je dirai 1'unique difficulte, qu'on cut elev6e contre le systeme des ondes, 

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

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

 blable et servir a rendre compte de 1'inegalite de refraction de ces ray- 

 ons ou de la dispersion. On a montre r6cemment que cette difBculte 

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

 inegalement denses de maniere que des rayons a ondulations dissem- 

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

 les conceptions des geometres a cet egard sont conformes a la nature 

 des choses. Voici les amplitudes des ondulations deduites experimen- 

 talement d'une serie de fails relatif aux interferences : 



mm. 



Violet 0-000423 



Jaune 0-000551 



Rouge 0-000620 



La vitesse de transmission des rayons de diffe>entes couleurs dans le 

 espaces celestes est la meme dans le systeme des ondes et tout-a-fait 

 Ind^pendante de 1'etendue ou de la vitesse des ondulations." 



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

 of different colors, having unequal undulations, must nevertheless be 

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

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

 which are transmitted through the atmosphere with equal velocity. 

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

 strated experimentally by the uniformity of effect produced by music 

 at all distances from the source whence it emanates. The principal, I 

 may say the only objection, advanced against the 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 thd "^fraction of the 

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

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

 transmitted through bodies of unequal density in such a manner that 

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

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

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

 ual nature of things. The following are the lengths of the undulations 



