on the Velocity of Light. 547 



two opposite theories regarding the nature of light be submitted 

 to a definite test ? We have adojited such measures as are cal- 

 culated to exhibit in a striking manner the differences of the 

 phsenomena as deduced from the one or the other theory. 



As remarked in our preceding communication^ the observation 

 was made simultaneously on two bundles of light ; the one having 

 traversed the air, the other a column of water. 



For each of these bundles the path was as follows : — A tele- 

 scope was so disposed that its object-glass was very near the 

 rotating mirror; a little rectangular prism was placed in the 

 focus of the telescope, in such a position that the solar rays fall- 

 ing upon it from a convenient lateral opening near the eye-glass, 

 were totally reflected towards the object-glass. 



Beyond the rotating mirror, and at a distance which for the 

 ray that passed through water amounted to two metres, there 

 was a fixed reflector designed to send back the light to the rota- 

 ting mirror by a normal reflexion. 



The focal distance of the telescope was such that the image of 

 the little prism placed at its focus formed itself distinctly upon 

 the fixed reflector just mentioned. After having been reflected 

 fi'om it, the light retm-ned to the rotating mirror, was sent on 

 thi'ough the telescope, and on passing the focus formed an image 

 which exactly covered the prism. 



By the rotation of the mirror we give birth to a number of 

 images which succeed each other very rapidly, and the super- 

 position of which produces the sensation of a pemianent image. 



When the rotation became sufficiently rapid, the permanent 

 image was pushed forward in the direction of rotation, this de- 

 viation being the result of the angular motion of the mirror 

 during the time occupied by the light in passing twice over the 

 space which separated it from the fixed mirror. 



A second similar fixed mirror was placed beside the former : it 

 permitted us to make the experiment with air and water simul- 

 taneously. 



If the lengths traversed had been equal for both media, the 

 times occupied in passing them would be in the ratio of 4:3 or 

 of 3 : 4, accordinir to the one or the other theory, and the devia- 

 tions produced by the rotation of the mirror would have been in 

 the same ratio. 



Instead of equal lengths we have adopted equivalent lengths ; 

 that is to say, lengths traversed by the light in equal times. 

 These lengths arc very different, according as they are calculated 

 from the one or tlie other theory. The length for water being 1, 



the equivalent length for air would be - by the theory of emis- 

 sion, and 77 by the theory of undulation. 



