384 Prof. D. B. Brace on the 



in one of which this coefficient is the ratio of the velocities 

 of light in quartz and in space, or approximately two-thirds. 

 The presence, however, of two first order terms might imply 

 still a zero first order effect if these terms completely com- 

 pensated one another. It is difficult, however, to see how this 

 could be ; and hence, from such a mode of reasoning, we might 

 still expect to find a residual first order effect which would be 

 represented by the aberration term with a small coefficient, 

 and which even the refined experiment of Rayleigh could not 

 show. In a later paper, Lorentz"*, in a reply to a criticism 

 of Larmorf, gives the relation of the action between two 

 elements arising from the electric forces which must obtain 

 in order that the earth's motion may not influence the rotation 

 of the plane of polarization to the first order. As this is not 

 the only possible mode of action, and as further it is only 

 true when second order terms are neglected, a definite 

 conclusion can only be reached by direct experimental 

 examination. 



While the following test does not fully attain a second 

 order sensibility, it does establish the absence of first order 

 terms, or, at least a compensation up to one part in five 

 hundred and probably to one part in one thousand. Instead 

 of quartz, which both Mascart and Rayleigh used, I employed 

 finally an active liquid, the oil of caraway-seed (a D = + 103°33' 

 per decimetre). Although its rotary power is much less than 

 quartz (a D = 21°*67 per millimetre), I found it preferable in 

 the arrangement used. Both Mascart and Rayleigh point 

 out some of the difficulties in such an experiment, involving 

 as it does such enormous rotations. Thus, with the five pieces 

 of quartz which Rayleigh used, the total rotation for sodium 

 light was more than 5500°, and this would give a difference 

 of rotation for the two D-lines of 1 1°, thus making the use 

 of such a source impossible. He actually used the yellow 

 helium line. Even then the field in his half- shade plane 

 polarizer was " decidedly inferior to that obtainable when the 

 quartzes were removed." This inferiority due to residual 

 light in the field seems to have originated more largely in 

 imperfections in the quartzes themselves. We should infer 

 from this that, even if the dispersion due to the actual 

 rotation were compensated for, the field would, on account of 

 such imperfections, be far less dark than that attainable with 

 a perfectly homogeneous substance. This was borne out by 



* Amsterdam Akad. v. Wet. March 29, 1902, p. 669. 

 t ' JEther and Matter/ Cambridge, 1900, pp. 214-215 ; also Phil. Mag. 

 Sept. 1902, p. 367. 



