JEtlier "Drift " and Rotary Polarization. 385 



my own experience in attempting to use quartz successfully 

 in the experiment. 



It is well known that if the direction of propagation of a 

 ray in an active substance is changed by a single reflexion, 

 the rotation is completely compensated for after passing over 

 the same distance. If such a compensation could be brought 

 about and still allow the influence of the earth-motion to be 

 unaffected^ we could use white light, and hence approximate 

 to the normal polariscopic sensibility of say 0°'01 or less, a 

 sensibility usually some ten times that attainable with homo- 

 geneous light from the same radiant under like conditions. 

 My first plan was to mount two equal lengths of the active 

 substance (in this case, quartz cylinders) at right angles to 

 each other, with one in the direction of drift, and then rotate 

 the system through 180°. This arrangement would give 

 complete compensation if the details of the system could be 

 realized. As this plan involved a change in the direction of 

 the ray of 90°, reflecting surfaces were necessary ; and since, 

 with white light, the azimuths of the various vibrations would 

 be at different incidence on the reflecting surface, the 

 rotations produced by this surface would, in general, follow 

 a different law from that of quartz, so that, to this extent, 

 compensation would be prevented. If, however, these rays 

 should all strike a second surface at the same incidence, but 

 in azimuths which are the complements of those at the first 

 surface, the effect due to reflexion would be completely com- 

 pensated for. This of course may be realized by making the 

 planes of incidence at ri^ht angles to one another. Thus, if 

 the plane of the paths within the active media is horizontal, 

 the plane of incidence of the first mirror may be vertical, 

 for example, and the ray be sent upward and then horizontally 

 from the second mirror, the plane of incidence of the latter 

 being also vertical but in an azimuth of 90° to the incident 

 plane of the first. Since now a single reflexion reverses an 

 axial displacement, i. e., a right-handed to a left-handed one, 

 a second reflexion would reverse this to a right-handed one 

 again, thus restoring the relative order, and so on. An odd 

 number of reflexions therefore reverses the relative axial 

 displacement, while an even number restores it* This 



* Mascart, in his experiment, realized a reversal by means of a half- 

 wave plate, following the method of Fizeau and Foucault, who used 

 the two parallelepipeds of Fresnel instead. He was in this way able to 

 use consecutively R- and L-quartz, their effects being added together. 

 The tcftal rotation which he thus obtained was approximately 6300° for 

 the green thallium line. This rotation is somewhat greater than that 

 obtained by Rayleigh, but the former's polariscopic sensibility was several 

 times less, depending, as it did, on the appearance or disappearance of 

 the thallium line as seen with a spectroscope between crossed nicols. 



