12 PROPAGATION OF LIGHT. 



of this objection. It may be supposed that the molecules of 

 light are originally projected with 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. 



The uniform velocity of light is, on the other hand, an 

 immediate consequence of the principles of the wave-theory. 

 It follows from these principles, that the velocity with which 

 /vibratory movement is propagated in an elastic medium de- 

 pends in no degree on the exciting cause, but varies solely 

 with the elasticity of the medium and its density. If these 

 then be supposed to be uniform in the vast spaces which in- 

 tervene between the material bodies of the universe, the ve- 

 locity will be the same, whatever be the luminous origin. 



(15) The rectilinear motion of light has long been urged 

 in favour of the theory of emission, and against the wave- 

 theory. If light consists in the undulations of an ethereal 

 medium (it has been said), as sound consists in the undulations 

 of the air, it should be propagated in all directions from every 

 new centre, and so bend round interposed obstacles. Thus 

 luminous objects should be visible, even when an obstacle is 

 between them and the eye, just as sounding bodies are heard, 

 though a dense body may be interposed between them and 

 the ear, and shadows could not exist. 



To this objection, which was that chiefly urged by New- 

 ton himself, it might be enough to reply, that though vibra- 

 tory motion in an elastic medium is propagated in all direc- 

 tions from every new centre, yet there is no reason to conclude 

 that it is propagated with the same intensity in every direc- 

 tion, however inclined to that of the original wave. In fact, 

 analogy furnishes grounds for an opposite conclusion ; for 

 there are a multitude of facts which prove that sound is not 

 propagated with the same intensity in all directions, however 



