1894.] Section-plates and Prisms of Artificial Crystals. 109 



circle-plate, whose power-arms are weighted, and whose short, curved 

 arms terminate in blunt knife edges which press upwards against a 

 collar fixed to the axis. The levers may be thrown out of action 

 when desired, and the weight of the axis added to by placing small 

 shot or weights in a brass cup carried at the upper end of the axis. 

 This arrangement enables the weight above the crystal to be modi- 

 fied to any extent. Within the second axis slides without rotation 

 the third one, which carries at its lower end the crystal and its means 

 of adjustment. The upper portion of this axis is tapped with a fine 

 screw thread, and the axis can be raised or lowered by a correspond- 

 ing milled nut. A collimator and telescope, arranged in a horizontal 

 plane at a suitable height for observing the crystal, are provided ; 

 they are movable over circular guiding arcs, whose centre lies in 

 the vertical axis of the instrument. Just below the plane of the 

 optical tubes the horizontal grinding disc is supported in a friction- 

 less bearing carried by an adjustable tripod. It is capable of rota- 

 tion by a suitable whirling apparatus arranged equally on each side 

 of the axle so as to minimise the strain of the band. The disc is of 

 moderately finely-ground, truly plane plate-glass, suitably mounted in 

 a brass frame, and lies upon a second, from which it is readily de- 

 tachable, so finely ground as to be almost transparent, and which is 

 employed exclusively for giving a final polish to the surface ground 

 by the removable disc. The grinding is under best control when the 

 driving pulley, which is provided with a suitable handle, is rotated 

 by hand, each revolution producing two revolutions of the grinding 

 disc. 



The means of adjusting the crystal so that any desired direction in 

 it may be brought perpendicular to the grinding surface are afforded 

 by two circular motions in the form of movable segments rotated by 

 tangent screws, which, in addition to a pair of centering movements, 

 are carried at the lower extremity of the inner vertical axis, and the 

 common centre of which is occupied by the crystal. These circular 

 movements are graduated to read directly to degrees, and easily by 

 estimation to ten minutes. Two interchangeable pairs of them are 

 provided ; the planes of motion of one pair are fixed at right angles, 

 while the planes of the other pair, designed for use in more compli- 

 cated cases of symmetry, may be adjusted at any desired angle to 

 each other by means of a small graduated horizontal circle carried by 

 one of them. 



The crystal is attached to the small cross-grooved disc of the 

 crystal holder by a hard and rapidly-setting wax. It is adjusted so 

 that the zone of faces perpendicular to which it is desired to grind a 

 surface, if such a zone is developed, is parallel to the vertical axis. 

 If such a zone is not present, as will happen in monoclinic and tri- 

 clinic crystals, a prominent zone, whose position with respect to the 



