388 Prof. D. B. Brace on the 



optic axis, and their ends polished in order to make a pre- 

 liminary examination of their optical qualities. Many of 

 these showed traces of the double spiral over various points 

 of their end faces, indicating the presence of both R- and L~ 

 formations to a greater or less extent, so that perhaps less 

 than a third of the cuttings could be used. The entire lot 

 was finally sent to an expert cutter, who declined to under- 

 take, with this material, the making of cylinders which should 

 give the optical conditions necessary for realizing the normal 

 polariscopic sensibility. Cylinders one-fourth as long, built 

 up from sections some 5 cm. in length of purer material 

 from Swiss sources, were proposed. Even then the final 

 outcome as to the polariscopic sensibility was much in doubt, 

 while the importance of the extension of this test did not 

 seem to me to warrant the great expense which the proposal 

 contemplated. The use of quartz was finally abandoned for 

 other available active materials. These I sought among the 

 active oils. 



The success of the same plan, as proposed for quartz, 

 seemed somewhat doubtful, since we should need two R- and 

 L-substances which for suitable lengths — not necessarily 

 equal — would give identical rotary dispersions. With the 

 great rotations proposed, any slight relative irrationality 

 would at once make itself evident by the corresponding 

 increase of the residual light in the field of the half-shade, 

 which would present the same order of difficulty as that from 

 impure quartz. Such oils would also have to be very clear 

 indeed to allow the passage of sufficient light through the 

 much greater distances necessary to produce the same total 

 rotation as in quartz. Several of the commercial oils fulfil 

 these conditions to a greater or less extent. Two of these, 

 caraway oil, a D = + 103°33', and eucalyptus oil,a D = — 52'22 7 ', 

 as prepared by Schimmel & Co., Miltitz-Leipzic, are quite 

 colourless and suited to the above arrangement. The expense 

 of the latter (8 marks per pound) precluded its use, but the 

 former (1 mark per pound) was entirely available, if the 

 optical conditions referred to above could be met by a single 

 substance. It did not at first occur to me that an arrange- 

 ment for testing the " drift " was possible with a single sub- 

 stance which would allow the use of white light, thus giving 

 the normal polariscopic sensibility. Both of the previous 

 experimenters had failed to make use of this idea. If the 

 motion of the earth produces an effect on the rotation of the 

 plane of polarization, we should of course expect this effect 

 to be reversed on changing the direction of motion. Thus 

 the plane of a ray propagated along the drift would show a 



