REFLEXION AND REFRACTION. 33 



and is therefore constant. But there is this important diffe- 

 rence between them : in the wave-theory, the sines of these 

 angles are in the direct ratio of the velocities, while, accord- 

 ing to the theory of emission, they are in the inverse ratio. In 

 other words, the velocity of light in the denser medium is 

 less according to the former theory ; while, according to the 

 latter, it is greater. Here, then, the two theories are directly 

 at issue upon a point of fact, and we have only to ascertain 

 how this fact stands, in order to be able to decide between 

 them. The important experiment by which this was first 

 accomplished was made by Arago; and the result, as will be 

 shown hereafter, is conclusive in favour of the wave-theory. 



(41) The conclusion deduced from the experiment here 

 referred to presupposes the laws of Interference of Light 

 laws which, in themselves, are intimately connected with the 

 principles of the wave- theory. It was desirable, therefore, to 

 deduce the same conclusion, if possible, by direct means. The 

 experiment by which this is effected has been made by 

 M. Foucault, upon a method devised by Arago ; its principle 

 will be understood from the following description. 



Let a ray of light, reflected by a heliostat, be admitted 

 into a darkened chamber in a horizontal direction, and fall 

 upon a mirror which revolves about a vertical axis situated 

 in its own plane. It is manifest that, as the mirror revolves, 

 the reflected ray will move in the horizontal plane passing 

 through the point of incidence, with an angular velocity 

 double of that of the mirror itself. Now, in this plane let a 

 second mirror be placed, perpendicular to the right line 

 joining the centres of the two mirrors. Then, when the ray 

 reflected by the revolving mirror meets the fixed mirror, in 

 the course of its angular movement, it will be turned back 

 on its course, and, after a second reflexion by the revolving 

 mirror, return towards the aperture. 



D 



