VELOCITY OF LIGHT. 113 



moving presents the same index of refraction as the light of those 

 from which it is receding. Using the language of the emission 

 hypothesis, this celebrated observer remarks, that bodies send 

 forth rays of all velocities, but that among these different velo- 

 cities one only is capable of exciting the sensation of light. 55 



this difficulty is not insurmountable, and that the ether 

 may be supposed to be 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 actual 

 nature of things. The following are the lengths of the undu- 

 lations, as experimentally deduced from a series of facts in 

 relation to interference : mm 



Violet 0-000423 



Yellow 0-000551 



Eed 0-000620 



The velocity of the transmission of rays of different colours 

 through celestial space, is equal in the system of waves, 

 and is quite independent of the length or the velocity of 

 the undulations." Arago, MS. of 1849. Compare also the 

 Annuairepour 1842, pp. 333-336. The length of the lumi- 

 nous wave of the ether, and the velocity of the vibrations, 

 determine the character of the coloured rays. To the violet, 

 which is the most refrangible ray, belong 662, while to the 

 red, (or least refrangible ray with the greatest length of wave,) 

 there belong 451 billions of vibrations in the second. 



55 " J'ai prouve, il y a bien des annees, par des observations 

 directes que les rayons des etoiles vers lesquelles la Terre 

 marche, et les rayons des etoiles dont la Terre s'eloigne, se 

 refractent exactement de la meme quantite. Un tel resultat 

 ne peut se concilier avec la theorie de remission qu'a Taide 

 d'une addition importante a faire a cette theorie : il faut ad- 

 mettre que les corps lumineux emettent des rayons de toutes 

 les vitesses, et que les seuls rayons d'une vitesse determinee 

 sont visibles, qu'eux seuls produisent dans 1'ceil la sensation 

 de lumiere. Dans la theorie de 1'emission, le rouge, le jaune, 

 le vert, le bleu, le violet solaires sont respectivement accompag- 



VOL. III. I 



