During the progress of the X-ray research on this substance by Miss Yardley, the results of which are embodied in the preceding communication, the need for a thorough reinvestigation of the crystallographic and especially the optical properties of the substance, was made very clear. The difficulty as regards the optical constants was that of preparing the properly orientated and absolutely truly plane-surfaced section-plates and 60°-prisms of this relatively soft and fragile substance, and after many attempts of a really heroic nature Miss Yardley appealed to the author, who was only too pleased to bring into service his cutting-and-grinding goniometer, with which the necessary plates and prisms were successfully prepared. The author, however, becoming very interested in the substance, has carried out also a complete goniometrical remeasurement, in the light of the new and correct setting which Miss Yardley has indicated, and a determination of the density of the crystals, so that with the optical data, now given for the first time, a revised and much more complete crystallographic description is herewith given. It agrees in every respect with the conclusions of Miss Yardley, as described in the immediately preceding communication. The only real difficulty met with, which renders the substance unusual to deal with, is the fact that no cementing material has been found which is without action on iodo-succinimide. For instance, Canada balsam and its solution in benzene (a hydrocarbon which, very fortunately, does not act on iodo-succinimide) or turpentine at once turn deep brown to black when used to cement section-plates to cover-glasses or to the little circular plates of parallel glass which the author uses in the preparation of the second surface. So that, after the first surface of a section-plate has been ground, the crystal being held as usual on the crystal holder by opticians’ hard wax, any action not mattering much here, the usual process—of cementing the crystal by this first surface on to one of the little glass discs, fitting the latter into the receptacle for it in the parallel-plate, preparing fitting of the cutting-and-grinding goniometer, and grinding the second surface without further trouble and thus making a plate as thin as may be desired—cannot be employed with iodo-succinimide. To prepare the second surface the crystal must be reset goniometrically, while held in a grip-holder instead of the usual holder carrying opticians’ wax, and then ground and polished with great care to avoid slipping in the gripper. As the substance is soft and friable this is rather a nuisance, especially, moreover, as it was desirable to be able to obtain section-plates of considerable tenuity for the purposes of Laue photographic X-ray analysis, as well as very thick ones for the study of possible optical activity. Happily, however, all these unusual difficulties were eventually overcome, and the desired results obtained.
Measurement of Wavelength and Temperature-Dependent Optical Properties of Thermochromic Pigments