670 Intelligence and Misd^llaTieom Articles, 



of rays thus penetrated into each tube, and traversed its entire 

 length, l"»-487. .\ ■vyyuu ^i^-iV- o 



The two bundles, always parallel to each other, reachfed the ob- 

 ject-glass of the second telescope, were then refracted, and by the 

 eiFect of the refraction reunited at its focus. There they encoun- 

 tered the reflecting plane of a mirror perpendicular to the axis of the 

 telescope, and underwent a reflexion back again towards the object- 

 glass ; but by the effect of this reflexion the rays had changed their 

 route in such a way that that which was to the right before, was to 

 the left after the reflexion, and vice versa. After having again passed 

 the object-glass, and been thus rendered parallel to each other, 

 they penetrated a second time into the tubes ; but as they were 

 inverted, those which had passed through one tube in going passed 

 through the other on returning. After their second transit through 

 the tubes, the two bundles again passed the double chinks, re-entered 

 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 maintained, 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 refraction, as if they were nearer to each other than they 

 really are. It is in this way possible to give various dimensions to 

 the fringes, and to choose that which is the most convenient for ob- 

 servation. 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 tempera- 

 ture or pressure between the two tubes, from which might result a 

 displacement of the fringes, which would be mingled with the dis- 

 placement 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 ; 

 60 that any alteration of the density in any point whatever of the 

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

 sequently have any influence upon the position of the fringes. The 

 compensation 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 posi- 

 tion 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. 



