124 ANNUAL OF SCIENTIFIC DISCOVERY. 



the tubes, the two bundles again passed the double chinks, reentered 

 the first telescope, and lastly intersected at its focus in passing across 

 the transparent mirror. There they formed the fringes of interference, 

 which were observed by a glass carrying a graduated scale at its focus. 

 It was necessary that the fringes should be very large, in order to be 

 able to measure the small fractions of the width of a fringe. I have 

 found that that result is obtained, and a great intensity of light main- 

 tained, by placing before one of the chinks a thick mirror, which is 

 inclined in such a way as to see the two chinks, by the effect of refrac- 

 tion, as if they were nearer to each other than they really are. It is 

 in this way possible to give various dimensions to the width of the 

 fringes, and to choose that which is most convenient for observation. 

 The double transit of the light was for the purpose of augmenting the 

 distance traversed in the medium in motion, and further to compensate 

 entirely any accidental difference of temperature or pressure between 

 the two tubes, which would be mingled with the displacement which 

 the motion alone would have produced, and thus have rendered the 

 observation of it uncertain. 



^ It is, in fact, easy to see that, in this arrangement, all the points 

 situated in the path of one ray are equally in the path of the other ; 

 so that any alteration of the density, in any point whatever, of the 

 transit, acts in the same manner upon the two rays, and cannot conse- 

 quently have any influence upon the position of the fringes. The com- 

 pensation may be satisfactorily shown to be complete, by placing a 

 thick mirror before one of the two chinks, or as well by filling only one 

 of the tubes with water, the other being full of air. Neither of these 

 two experiments gives rise to the least alteration in the position of the 

 fringes. With regard to the motion, it is seen, on the contrary, that 

 the two rays are subject to opposite influences. If it is supposed that 

 in the tube situated to the right the water runs towards the observer, 

 that of the two rays which comes from the right will have traversed 

 the tube in the direction of the motion, while the ray coming from, 

 the left will have passed in a direction contrary to that of the motion. 

 By making the water move in the two tubes at the same time, and in 

 contrary directions in each, it will be seen that the effect should be 

 doubled. This double current having been produced, the direction 

 may be again reversed simultaneously in the two tubes, and the effect 

 would again be double. 



All the movements of the water were produced in a very simple 

 manner, each tube being connected by two conduits, situated near 

 their extremities^ with two reservoirs of glass, in which a pressure is 

 alternately exercised by means of compressed, air. By means of this 

 pressure the water passes from one reservoir to the other by traversing 

 the tube, the two extremities of which are closed by the mirrors. The 

 interior diameter of the tubes was five millimetres, their length one 

 metre and four tenths. The tubes were of glass. The pressure under 

 which the flowing of the water took place might have exceeded two 

 atmospheres. The velocity was calculated by dividing the volume of 

 water running in one second^ by the area of the section of the tube. 

 Great care was taken to obviate the effects of the accidental motions 



