NORMAL SULPHATES OF POTASSIUM, RUBIDIUM, AND CH5SIUM. 
459 
the centering movements of the crystal-adjusting apparatus, and thus avoid concentric 
grooving of the grinding surface, besides no longer recpiiring the centering of the 
crystal to be disturbed. This is achieved by mounting the grinding gear on a slider^ 
Avhich can be made to traverse a bevelled bed resting on the base of the instrument, 
by rotation of a long screAA^ gearing Avith a corresponding thread in the bed and 
manipulated by a Avinch handle. The gunmetal bed is 20 centims. long, 6 centims. 
Avide at the top surface, the greater portion of AAdiich is planed out into a depression 
so as to reduce friction by only leaAung narroAv strips along the long edges to act as 
guides for the slider, and 4 centims. Avide at the base, there being thus a bevel of 
1 centim. at each side. The bed is fixed firmly to tlie main base of the instrument l^y 
four large screAvs. At the outer end is a cup-shaped holloAv ending in the tapped 
horizontal holloAV cylinder of steel vvhich gears Avith the steel screAv carried by the 
slider. The latter is slightly longer than the bed, and 8 centims. broad. It is a 
solid casting of steel holloAved underneath to the shape of the bed; the thickness 
above the bed is 8 millims., and the sides, bevelled on their interior, haAm a depth of 
24 miUims. In order to ensure close but not iiicoiiAmniently tight fitting of the slider 
on its bed, room has been left on one side for the insertion of a thick strip of steel 
l^etAveen the bevelled edges of the slider and the bed, and this is attached to the 
slider by five screAA^s Avhose heads are on its outside, and Avhich serve as adjusting 
screAvs to regulate the fitting of bed and slider. At the outer end the slider narrows 
off laterally and its termination is deepened to the depth of the bevelled sides, so as 
to form a stout support for the passage of the thick steel cylinder which, beyond this 
bearing, Avhere it is flanged to prevent its traversing, is cut Avith a deep screAV-thread 
corresponding to the hollow one in the bed. Immediately outside this bearing, the 
steel cylinder passes into the brass Avinch fitted Avith ebonite handle. As the other 
end of the screAv is unhanged, the slider can Iac readily removed from, or re-attached 
to, the bed ; on pushing the slider OA^er the bed it slides unimpeded for a dozen 
centims., Avhien tlie screAv begins to gear, and further movement is effected by the 
AAuncli. The grinding gear is similar to that permanently hxed to the base in the 
previous instrumeiit, the grinding table being provided Avith an efficient means of 
adjustment exactly perpendicular to the vertical goniometrlcal axis, and the friction 
pulley Avith a means of adjustment for tlie tightening of the driving band. The 
former is necessarily effected differently to the method employed in the previous 
instrument, as it is no longer possible to penetrate the base for the utilisation of the 
spiral spring and tripod method. It is here effected by an adjustable stout circular 
base-plate, Avhich is fixed to the slider by six screAA^s, three of AA^oich penetrate both 
plate and slider, and three pass through the plate only and thus act as adjusting- 
screws ; after these are adjusted the table is fixed m the adjusted position by means 
of the other three screws. The adjustment of the friction pulley is effected by 
carrying it on a Iwacket AA-hich is fixed at the inner end of the siider ])y tAvo Inroad- 
headed screAA's, Avorking in slots of the recpiired length in the horizontal claAA-s of the 
3 N 2 
