THE VELOCITY OF LIGHT. 219 



deal was still left, which it was desirable to experiment 

 upon. I have told you that the question of the velocity of 

 light was able to satisfactorily settle the question between 

 the corpuscular theory of light as enunciated by Newton, 

 and the undulatory theory. How was that possible 1 It 

 depended on the explanation of the theory of refraction. 

 The refraction of a ray of light when it falls on the 

 surface of a dense medium, is such that it is bent down- 

 wards towards the normal, as you know. In order to 

 explain this, Newton supposed those particles of matter, 

 the corpuscles, were attracted downwards towards a dense 

 medium, and that therefore as soon as they came within 

 the sphere of attraction of this medium, they came with 

 greater velocity downwards, and so in passing through the 

 medium they were deflected downwards, and also went 

 with greater velocity. The explanation afforded by the 

 nndulatory theory, on the other hand, assumes that in a 

 dense medium the velocity of light was less than in a 

 light medium such as air, consequently it was pointed out, 

 by Arago especially, that a determination of the relative 

 velocity of light in air and water would be a conclusive 

 crucial experiment to settle which of these two theories 

 was true. At the time Arago pointed this out, the late 

 lamented Mr. Wheatstone had just employed a most 

 ingenious apparatus, the revolving mirror, in the deter- 

 mination of the velocity of electricity. Here we have a 

 mirror belonging to the Paris Observatory, founded, I 

 presume, simply on Wheatstone's model. It is a piece of 

 mechanism designed with great skill by M. Breguet, the 

 accomplished French mechanician, in order to give a great 

 velocity of rotation to this little mirror. Wheatstone had 

 used this mirror revolving with very great velocities. You 

 can easily get a thousand revolutions in a second, and if 

 you have such a velocity, and there is a beam of light 

 falling upon it, then the reflected beam will be turned 

 through an angular distance with very great velocity ; and 

 Arago proposed that a beam of light should be thrown on 

 the revolving mirror, and then should be sent a little 

 distance and reflected back again upon it. The mirror 

 having been in that time rotated through a small angle, 

 the reflected ray would be displaced through twice the 

 angle through which the mirror was rotated. If the mirror 



