April 25, 1895] 



NA TURE 



609 



Prof. Klein, that this has been arranged for in the new 

 Fuess instrument. The carriers of the nicols are each 

 furnished with a toothed flange capable of gearing with 

 a smaller pinion, and the two pinions are arranged at 

 the ends of a connecting rod furnished at a convenient 

 height near the upper pinion with a milled flange by 

 means of which rotation can be effected. I'rovision is 

 made for the lengthening of the connecting rod when the 

 focussing of the microscope by the rack and pinion or by 

 the fine adjustment is effected, and care is also taken 

 that the rotation by means of the connecting rod shall 

 occur without dead-space or backlash. Prof Klein states 

 that some important details in connection with improve- 

 ments in the mode of carrying out this simultaneous 

 movement of polariser and analyser will shortly be pub- 

 lished by Herr Fuess. Provision has likewise been made 

 for correcting at any time the setting of the nicols in 

 their carriers, experience having shown that the setting 

 invariably alters slightly in course of time. In addition 

 to the eyepiece nicol capable of being connected with 

 the polariser in the manner just described, there is like- 

 wise provided the usual nicol capable of sliding in or out 

 of the microscope tube just over the objective. ."Xbove 

 this, and just below the eyepiece, a Bertrand lens for 

 observing interference figures in convergent light is 

 capable of sliding in and out of the tube, and is intended 

 to be employed in conjunction with a converging system 

 of lenses capable of being carried in a tube attachment 

 beneath the level of the stage. The remaining details 

 of the microscope are the same as are usually supplied 

 with the No. I Fuess instrument. 



The stage goniometer is intended to be employed with 

 the microscope arranged horizontally, as it is found 

 mconvenient to employ an immersion liquid with a ver- 

 tical arrangement. The base-plate of the goniometer, 

 consisting of a stout metal plate with fairly large central 

 aperture, is fi.ved by a suitable clamping arrangement 

 upon the now vertical stage of the microscope. The 

 plate is continued into a short arm on that side which 

 is uppermost when fixed in position, and this arm carries 

 near its end, and at right angles to it (horizontal when 

 in position), a projecting piece terminating in the sup- 

 porting cone for the goniometer circle, and which also 

 carries the vernier reading to five minutes and the fine 

 adjustment. The circle is hollowed in its upper central 

 part, and perforated with a central aperture ; this permits 

 of the sliding movement within the hollow, for centering 

 purposes, of a disc which carries the axis of the instru- 

 ment. To the lower end of this short axis are attached 

 the movements for adjusting the crystal, and the lower 

 of which carries the crystal. The adjusting movements 

 are a pair of circular quadrants arranged at right angles 

 to each other and graduated. They are simpler in con- 

 struction, and lie much closer together than those of the 

 best forms of goniometer now in use for ordinary gonio- 

 metric and spectrometric work, and are thcreloie par- 

 ticularly suitable for use in connection with the micro- 

 scope. The upper quadrant is fixed to the axis ; over it 

 a slider is capable of moving, which carries a vernier, 

 and below it the lower quadrant, which in turn is fitted 

 with a slider terminating in the holder which carries 

 the crystal cemented by wax. The verniers enable read- 

 ings of five minutes to be obtained, the same degree of 

 accuracy as in the case of the circle. 



The glass cell containing the immersion liquid is sup- 

 ported in position normal to the axis of the microscope 

 by means of a stand with an adjustable aim placed to the 

 left of the microscope. It is recommended to have a 

 series of cells, ready for filling with various media of the 

 most frequently required refractive power. The advan- 

 tages of Adams' method of determining optic axial 

 angles may also be combined with those of the method 

 now described, by use of a cell consisting of an upper 

 cylindrical portion terminating below in a sphere filled 



NO. 1330, VOL. 51] 



with the liquid. As regards suitable liquids, an admirable 

 list is given by Herr Pulfrich in his book descriptive of 

 the construction and use of the total-reflectometer 

 recently devised by him (p. 64). Two errors in that list, 

 however, are corrected by Prof. Klein ; he has been 

 unable to prepare the solution of mercuric iodide in ani- 

 line and quinoline of refractive index 22, and the refrac- 

 tive index of the phenyl sulphide kindly supplied by 

 Prof Klein's colleague, Prof Emil Fischer, is only r56 

 instead of r95. If the dangerously poisonous and in- 

 flammable liquids are excluded, the list consists chiefly 

 of oils, the well-known Thoulet solution, monobrom- 

 naphthalene, and methylene iodide. The solution of 

 iodine in the latter frequently renders it insufficiently 

 transparent for the purpose. 



The determination of the true angle, 2\', between the 

 optic axes within the crystal, supposing it to be biaxial, 

 can at once be determined with the aid of the new ap- 

 paratus, by immersing the crystal in a liquid whose re- 

 fractive index is equal to the H (the intermediate) 

 refractive index of the crystal. The condensing system 

 of lenses is first inserted between the polarising nicol and 

 the stage, and the Bertrand lens above the analyser ; as 

 objective, either the ordinary wide angle combination 

 usually employed for convergent light work, or a specially 

 constructed one supplied for the particular purpose of 

 convergent light observations through an immersion 

 liquid, is employed. This objective is so constituted 

 that as large a field of vision as possible is afforded, while 

 the distance between objective and crystal is considerably 

 greater than with the ordinary systems in use. The ap- 

 parent angle of the optic axes in air, 2F, may first be 

 measured, if desired, after adjustment of the crystal by 

 means of the adjusting movements, by bringing the 

 hyperbolic brushes to the cross wire of the microscope 

 eyepiece in the usual manner. The immersion cell not 

 being in position while this is being achieved, the ob- 

 jective can be approached nearer to the crystal and one 

 of the ordinary forms of convergent light objective em- 

 ployed, which affords a larger ^.ngle of vision, 

 reserving the special objective for the determination 

 of the true angle of the optic axes. If, however, 

 the Adams spherical cell is employed, there is no 

 necessity even here to use the special objective, as 

 the older wide angle form serves admirably. With 

 the parallel sided cells it is preferable to use the 

 special objective. The Adams sphere is not supported 

 similarly to the rectangular cells, but is conveniently 

 held by its cylindrical neck in a small support directly 

 attached to the lower ciuadrant of the adjusting apparatus. 

 The measurement of the true angle of the optic axes is 

 then carried out in the usual manner, similarly to the de- 

 termination of the apparent angle in air, while the 

 crystal is immersed in the liquid contained in one or 

 other of the two forms of cell. Monochromatic light 

 should of course be used in making the observations, 

 a sodium flame some little distance in front of the 

 polariser being employed by Prof. Klein. 



The great advantage of this method of determining 

 the true inner angle between the optic axes lies in the 

 fact that it is totally unnecessary to prepare section- 

 plates of the crystal, the whole crystal itself being em- 

 ployed, and thus material saved. Prof Klein does not 

 claim for it the highest attainable accuracy, and for the 

 class of work such as that with wliich the writer of this 

 article has become identified, the determination of the 

 crystallographic characters of series of isomorphous com- 

 pounds closely resembling each other, where every en- 

 deavour must be made to attain the upper limits of 

 experimental accuracy, such a method is of course 

 inadet|uate. But for the ordinary description of minerals 

 and the crystals of isolated chemical preparations 

 unlikely to be injured by the immersion liquid, and par- 

 ticularly for laboratory teaching, the method is one of 



