NORMAL SULPHATES OF POTASSIUM, RUBIDIUM, AND CH^SIUM. 
4(5:l 
is, in the case of the upper adjusting movement by slight rotation of the large segment 
about the upper fixed segment, the clamping screw being fixed, with the indicator on 
the carrier at zero. The screw is then unloosed and the slider, together with the lower 
segments carried by it, moved round 60° on one side, as indicated by the silver index 
on the carrier, which travels closely underneath the large divided arc. If the 60° are 
not conveniently attained by the hand movement of the carrier, the difference can be 
nicely made ujd by movement of the large segment about the ujDj^er arc by means 
of its tangent screw. If the original position of the upper segment had been noted 
on the silver scale it could readily be re-attained after the grinding of the first 
surface. When the latter has been achieved, the lower part of the apparatus is 
transferred to a position 60° on the other side of the centre, by a total sliding under¬ 
neath the large segment of 120°, and the second surface is ground. 
Another smaller but very useful addition is a spring-clutch to keep up the counter¬ 
balanced gun-metal axis, the apparatus for varying the pressure of the crystal on the 
grinding lap, when the left hand is removed from that one of the counterpoising levers 
which it manipulates during grinding. In order to prevent this axis from moving during 
the adjustmenf' of the crystal, a hooked spring-clutch is arranged about the bearing of 
the lever, in such a manner that when it is pushed over into position, it is maintained 
there by the force of a spring. It consists of an arm rotatable about an axis screwed 
into the lower part of one side of the bearing, and carrying a short horizontal bar 
attached at its outer end which j^resses down on the outer arm of the counterpoising 
lever when in position; a strong spring fixed to the base of the bearing and which 
has to be overcome on moving over the little arm, presses up against an angle of the 
latter below the axis in such a manner as to keep the bar of the clutch, after being 
brought over into position, firmly down on the counterpoising lever. The lever is 
thus fixed with its elbow resting on the circle plate, and its other arm carrying the 
knife edge is maintained pressed up against the collar of tlie gun-metal axis, and 
tlius the axis and the crystal which it carries at its lower extremity is unable to fall 
out of position during tlie adjustment. 
Tlie remaining improvements are two additions to the telescope of the goniometer, 
due to the suggestion of the author’s friend Professor H. A. Miers, who ])ad already 
had such additions made to the telescope of an inverted goniometer constructed tbr 
him by the same firm, Messrs. Troughton and Simms, and which wms intended for 
the study of the vicinal faces of crystals while in the act of growth in a cell of 
mother liquor. As the author’s cutting and grinding goniometer forms a most 
excellent inverted goniometer, it was considered advisable to adopt these additions. 
A rectangular cell with truly plane glass sides is also included to contain the satu¬ 
rated solution employed in such investigations. 
The first of the two consists of a combined goniometer- and micrometer-eyepiece, 
which provides two fixed spider-lines arranged at 90°, one vertical and adjusted 
exactly parallel to the vertical axis of the goniometer and the other horizontal, both 
