PROPAGATION OF LIGHT PRINCIPLE OF INTERFERENCE. 25 



as the position seems at first, he has very clearly shown that no 

 fair argument can be advanced against it, founded on the opacity 

 of the mass which the ether is supposed to permeate.* 



The discoveries of Bradley and Roemer, when compared toge- 

 ther, have led to a further and most important conclusion respect- 

 ing light namely, that its velocity is one and the same, whatever 

 be the luminous origin, the light of the sun, the fixed stars, the 

 planets and their satellites, being all propagated with the same 

 swiftness. This conclusion must be allowed to present a for- 

 midable difficulty in the theory of emission. Laplace has shown 

 that if the diameter of a fixed star were 250 times as great as that 

 of our sun, its density being the same, its attraction would be suf- 

 ficient to destroy the whole momentum of the emitted molecules, 

 and the star would be invisible at great distances, f With a 

 smaller mass there will be a corresponding retardation ; so that 

 the final velocities will be different, whatever be the initial. The 

 suggestion of M. Arago seems to offer the only means of avoiding 

 this difficulty. It may be supposed that the molecules of light are 

 originally projected with very different velocities ; but that among 

 these velocities there is but one which is adapted to our organs of 

 vision, and which produces the sensation of light. This supposi- 

 tion seems to be supported by the discoveries of Herschel, Wol- 

 laston, and Bitter, respecting the invisible rays of the spectrum ; 

 but it does not appear to be easily reconciled with any hypothesis 

 which we are able to frame respecting the nature of vision. This 

 uniformity of velocity, on the other hand, is a necessary conse- 

 quence of the principles of the wave-theory. The velocity with 

 which vibratory movement is propagated in an elastic medium de- 

 pends solely on the elasticity of that medium, and on its density ; 

 and if these be uniform in the vast spaces which intervene between 

 the material bodies of the universe and it is not easy to suppose it 

 otherwise the velocity must be the same, whatever be the origi- 

 nating source. 



The rectilinear motion of light has long been urged in favour 

 of the theory of emission, and against the theory of waves. If 

 light consists in the undulations of an elastic fluid, it has been said, 

 it should be propagated in all directions from every new centre, 



* " Sur 1' Influence du Mouvement terrestre dans quelqut-8 Phcnomenes d'Optiquo, " 

 Annales de C/iitnie, torn. ix. 

 t Zach, Ephem. t iv. 1. 



