VELOCITY OF LIGHT. 85 



refraction in the prism is not altered by the relation of the 

 velocity of light to that of the earth's motion. All the meas- 

 urements coincide in the result, that the light of those stars 

 toward which the earth is 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 celebra- 

 ted observer remarks, that bodies send forth rays of all ve- 

 locities, but that among these different velocities one only 

 is capable of exciting the sensation of light.* 1 



as experimentally deduced from a series of facts in relation to inter- 

 ference : 



mm. 



Violet 0-000423 



Yellow 0-000551 



Red 0-000620 



The velocity of the transmission of rays of different colors through ce- 

 lestial 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 Annuaire pour 1842, p. 333-336. The length of the 

 luminous wave of the ether, and the velocity of the vibrations, determ- 

 ine the character of the colored rays. To the violet, which is the most 

 refrangible ray, belong 662, while to the red (or least refrangible ray 

 with the gi-eatest length of wave) there belong 451 billions of vibra- 

 tions in the second. 



* " 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 ray- 

 ons des etoiles dont la terre s'eloigue, se refractent exactement de la 

 meme quantite. Un tel resultat ne peut se concilier avec la thiorie de 

 Vtmission qu'a l'aide d'une addition importante a faire a cette thedrie : 

 il faiit admettre que les corps lumineux emettent des rayons de toutes 

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

 bles, qu'eux seuls produisent dans l'oeil la sensation de lumiere. Dans 

 la theorie de l'emission, le rouge, le jaune, le vert, le bleu, le violet so- 

 laires sontrespectivement accompagnes de rayons pareils, mais obscurs 

 par defaut ou par exces de vitesse. A plus de vitesse correspond une 

 moindre refraction, comme moins de vitesse entraine une refraction plus 

 grande. Ainsi chaque rayon rouge visible est accompagne de rayons 

 obscurs de la meme nature, qui se refractent les uns plus, les autres 

 moins que ltd : ainsi il existe des rayons dans les stries noires de la por- 

 tion rouge du spectre ; la meme chose doit etre admise des stries situ 

 ees dans les portions jaunes, vertes, bleues et violettes." 



" I showed many years ago, by direct observations, that the rays of 

 those stars toward which the earth moves, and the rays of those stars 

 from which it recedes, are repeated in exactly the same degree. Such 

 a result can not be reconciled with the theory of emission, unless we 

 make the important admission that luminous bodies emit rays of all ve- 

 locities, and that only rays of a determined velocity are visible, these 

 alone being capable of impressing the eye with the sensation of light. 

 In the theory of emission, the red, yellow, green, blue, and violet so- 

 lar rays are respectively accompanied by like rays, which are, how- 

 ever, dark from deficiency or excess of velocity. Excessive velocity is 



