596 Prof. Brace on the Change produced by the 



placed upon the same pivoted trough, which I used before in 

 my experiments on the double refraction of water * and on 

 rotary polarization f . A seat was also fastened upon the 

 trough so that I could keep my eye uninterruptedly in a 

 fixed position E, before the telescope T, when the system was 

 rotated. The distances IM and MM' A were 40 cm, and 

 180 cm. respectively, and the angle MM' A was 10° approxi- 

 mately. The plates 1, 1', 2, 2', were selected from the ten 

 plates mentioned above, so as to give as far as possible the 

 best definition. The analyser was first set for a match, and 

 then plates 1 and 1' inserted, and the mirror M' and 

 analyser A adjusted so as to bring the image of the radiant 

 upon the centre of the latter. Plate 1' was rotated in its 

 own plane until the multiple images disappeared, and then 

 its carrier was turned in its vertical plane sufficiently to give 

 a match, the azimuth of the plates thus becoming the same 

 with respect to the direction of propagation of the ray at 

 incidence. Plates 2 and 2' were then inserted and similar 

 adjustments made. After various minor adjustments, 1 

 found that either half of the field could be made practically 

 black, on rotating the analyser, showing that the ray had 

 suffered but a slight amount of depolarization by the reflexion 

 from the mirror M' or by the transmission through the plates. 

 The vanishing line, however, was somewhat indistinct, but, 

 by diaphragming the aperture of the system down to about 

 6 mm. at L, I had a small portion of the field which was 

 fairly uniform with a moderately sharp vanishing line. With 

 carefully figured plates and a more intense source I might 

 have increased my sensibility considerably, but I found this 

 large enough for the purpose as it was. 



The magnitude of the probable effect Fizeau calculated by 

 means of Fresnel's " coefficient of convection " for the 

 " sether drift," namely, (fi 2 — l)//^ 2 - The change in jjl due to 

 the velocity v of the glass plate, the velocity of propagation 

 of the ray being V, becomes 



In order to find the corresponding change in the azimuth 

 of the ray due to such an effect, he compared the rotations of 

 two piles of plates of crown and flint glasses, respectively, 

 with the difference in their indices. Thus he found 



8 ^-«)_ 4-686^ 



* Phil. Mag. April 1904, p. 318. 

 t Phil. Mag. Sept. 1905, p. 383. 



