November ii, 1897 J 



NA TURE 



Z7 



a crystal with precisely such an oriental ion as mijjht be desired, 

 witli respect to the natural faces, and therefore to the morpho- 

 logical axes. For the whole of the optical investigation is 

 carried out by means of such plates and prisms of known orient- 

 ation, the former being requisite for the establishment of the 

 lx)sitions of the principal optical planes, the meajurement of the 

 optic axial angle, and the study of the interference phenomena, 

 while the latter are essential for the determination of the three 

 refractive indices. Manifestly, therefore, the accuracy of the 

 optical results depends primarily upon the precision with which 

 the desired orientation of these plates and prisms is attained. 

 Hitherto crystallographers who have investigated the optical 

 properties of the crystals of laboratory preparations, which are 

 so much softer and more friable than mineral crystals, have 

 been content either to employ plates or prisms formed by suit- 

 ably disposed natural faces of the crystals themselves ; or, failing 

 such, to prepare them by grinding the crystals in oil upon a 



ground-glass plate, the crystal being held between the finger and 

 thumb during the process. The difficulty of thus obtaining a 

 plane surface having the (desired orientation must be at once 

 apparent, to say nothing of the imminent risk of breaking the 

 crystal. At the best such a method can only be approximate, 

 and it is attended with so much that is troublesome and vexa- 

 tious that the investigation of a large series of compounds, in the 

 detailed and accurate manner which the author desired, would 

 be impossible. 



After much consideration an instrument was eventually de- 

 vised, and constructed for the author by Messrs. Troughton and 

 Simms, which achieves the desired object in a most satisfactory 

 manner. The author has no hesitation in ascribing the success 

 which has attended the inve.stigation to the admirable manner in 

 which this instrument performs its functions. More than five 



NO. 1463, VOL. 57] 



hundred plates and prisms have already been prepared by its aid 

 and employed in the work ; and never once, lor instance, has a 

 plate which was desired to be perpendicular to the acute bisectrix 

 of the optic axes failed to exhibit the interference figure by 

 convergent polarised light precisely .symmetrical to the centre of 

 the field, as it should be. Moreover, not more than half a dozen 

 crystals have been broken during grinding, and absolutely no 

 plates or prisms have required to be rejected on account of want 

 of accuracy of orientation. Further, what was by the old method 

 a most tedious and disagreeable part of crystallographical work, 

 now becomes one of the most delightful and interesting. 



The instrument will be found fully described in the Transac- 

 tions of the Royal Society (1894, A, 887), but a few words here 

 as to the principles of its construction may not be without 

 interest. It is represented, by the kind permission of the Royal 

 Society, in Fig. i. It may be succinctly termed a grinding 

 goniometer, for it combines an accurate, suspended, horizontal- 

 circle goniometer with a grinding apparatus. The telescope and 

 signal-collimator, together with the circle and the suspended 

 crystal-adjusting apparatus, form the goniometer. The segments 

 of the circular movements of the adjusting apparatus carry finely 

 graduated silver arcs, in order to enable the crystal to be set at 

 any angular position, with respect to any zone of faces previously 

 adjusted parallel to the axis with the aid of the telescope and 

 signal-collimator. The grinding apparatus consists of a small 

 finely ground glass disc, capable of being rotated by hand driving 

 gear. By a simple device the latter is made almost frictionless, 

 St) that the disc is rotated almost without effort. Three inter- 

 changeable discs are provided, the second being of extremely 

 finely ground glass, and the third of polished glass, these two 

 latter being employed for polishing ; all are used lubricated with 

 a thin film of oil. The axis of the instrument is capable of being 

 lowered or raised, so as to bring the crystal to the grinder or re- 

 move it, by means of a large milled-headed nut near the summit, 

 which engages with a screw thread on the upper part of the 

 inner axis. In addition to this, however, there is another outer 

 concentric axis capable of vertical movement, intended to enable 

 the operator to regulate the pressure with which the crystal bears 

 on the grinding surface, in order to avoid breaking the crystal. 

 This axis slides in the cylindrical bore of the rotatable cone which 

 carries the circle, and it is capable of being fully or partially 

 counterpoised by two weighted levers carried above the circle 

 plate. It is usually found most convenient to throw the back 

 lever out of gear during the grinding, by raising a screw pro- 

 vided for the purpose on the circle plate, thus leaving half the 

 weight of the axis to bear downwards when the front lever is 

 free to act, and then to more or less curtail the freedom of the 

 latter by gentle manipulation with the left hand, while the right 

 hand is used to rotate the driving pulley of the 

 grinding gear. After a little practice the pressure 

 is nicely regulated almost involuntarily by the left 

 hand in accordance with the " feel " of the grinding, 

 rendering it most unusual to crush the crystal oper- 

 ated upon. 



Continuous use of this instrument has proved it to 

 be all that can be desired for use with the crystals of 

 chemical preparations. A somewhat larger instrument 

 has since been constructed for the author, and was 

 described in the Proceedings of the Royal Society 

 (57, 324), for use either with chemical preparations or 

 with the harder crystals of minerals ; this instrument 

 includes an independent diamond-fed cutting ap- 

 paratus, and a large selection of metallic and 

 other grinding and polishing laps, so that plates 

 or prisms of even the hardest natural gems can be cut 

 and subsequently ground and polished in an equally satisfactory 

 manner. A duplicate of this instrument is included in the 

 National Collection in the South Kensington Museum. 



A further original piece of apparatus, which the author has 

 also found invaluable in the optical part of the work, is a spec- 

 troscopic monochromatic illuminator, a description of which will 

 be found in the Transactions of the Royal Society (1894, A, 

 913). It has enabled the author to make observations in every 

 ca.se for six wavelengths at suitable intervals in the spectrum, 

 and has proved particularly useful inasmuch as each of the series 

 of salts investigated has included at least one member which 

 exhibited exceptional optical properties, generally involving 

 crossed axial plane dispersion of the optic axes, in which in- 

 stances the command of an illuminator which could be made to 

 yield light of any desired wave-length enabled the phenomena 



