The Mther "Drift " and Rotary Polarization. 383 



very long sheets of ebonite: indeed that which I used had to 

 be specially rolled. But it is not necessary for the tubes and 

 sheets to be continuous; a succession of short lengths could 

 be employed. I think, however, that a practical limit must 

 exist to the total length of tube that can be advantageously 

 employed, for the definition was always better when the 

 tubes were removed, and was better with tubes 1 foot long 

 than with tubes 5 feet long, a fact which must, in part at any 

 rate, be attributed to reflexion of diffracted and scattered 

 light, at grazing incidence from the partition into the field of 

 the eyepiece. It was to diminish the effect of such reflexion 

 that in some experiments the slits D D were set wider apart 

 and the beams subsequently brought nearer together by the 

 paral] el-plate Jamin device. But the gain was apparently 

 more than compensated by the loss due to the dispersion, by 

 refraction, of each beam in traversing the Jamin plates, since 

 the light was not homogeneous. With a homogeneous source, 

 if one bright enough can be found, this would not happen. 



July 22, 1905. 



XLV. The Mther " Drift " and Rotary Polarization. 

 By J). B. Brace*. 



MASCARTf, some thirty-five years ago, first examined 

 the effect of the motion of the earth on double circular 

 refraction, using R- and L-quartz, from which he concluded 

 that the rotary power of quartz is not altered by the 2W00 

 part when the ray is reversed in the direction of motion of 

 the earth. 



Lord Rayleigh { recently repeated this experiment, also 

 using quartz, in which he found that such a reversal does not 

 alter the rotation by jo~oVo~o P ar ^. 



By using an active oil — the oil of caraway — and reversing 

 the ray so as to compensate for rotary dispersion, I have been 

 able by means of a sensitive-strip analyser, giving a much 

 higher accuracy, to carry the limit certainly twenty-five and 

 probably fifty times as far, so that we may conclude that the 

 effect of the motion of the earth on the rotation in active 

 substances is certainly less than 5^-oowo an( ^ probably less 

 than xo~ooWoo °^ ^he total rotation. 



Lorentz §, in his early analysis, gives two first order terms, 



* Communicated by the Author, 

 f Ann. de TEcole Normale, vol. i. p. 157 (1872). 

 X Phil. Mag. Aug. 1902, p. 215. 

 § Vevsuch einer Theorie, Leiden (1895), p. 138. 

 2 D 2 



