460 MR. A. E. TUTTON ON THE THERMAL DEFORMATION OF THE CRYSTALLISED 
bracket. Both this attachment and the circular-plate-bearing attachment of the 
driving pulley carried at the outer end of the slider can be slightly tilted if desired, 
so as to make the jDulleys I'un sufhciently eccentrically to avoid friction of the band at 
the two crossing points on each side of the grinding table pulley. The slider carries 
in addition, at the middle of the friction pulley end, a small brass plate supporting a 
short tube, in which, together with two holes driven into the slider one near each side 
and on the other side of the grinding table, fit three rod supports for a circular guard 
to surround the grinding table and prevent projection of the lubricating liquid when 
grinding. The guard has a wdndow on the side nearest the friction-jjulley, for the 
2 )assage of the crystal and its suppoi'ting apparatus as the slider is pushed into 
jiosition; the window can be closed by a shutter after the passage of the crystal, a 
strip of metal of similar curvature to the guard itself being fitted round this j^ortion 
of the guard for this purpose, and made movable with tlie aid of a little handle along 
a suitable slot directed by l)roadbeaded guiding pins. 
The height of the giinding surface of the lap, when the slider is in position, is 
exactly the same as that of the cutting disc when the cutting gear is in position, so 
that no variatioii of the height of the crystal-carrying axis is required. The cutting 
gear is exactly as in the formei instrument, rotatable about the back pillar, and 
siq)j)orted also, when in jiosition, in a traversing apparatus carried by the front pillar. 
The slider carrying the grinding aj^paratus is removed when the cutter is in position. 
In addition to the laps provided with the previous instrument, two additional ones 
are furnished. One is a polishing lap for hard crystals, consisting of hard opticians’ 
wrix melted into a circular metal tray of the same size as the other laps, and after¬ 
wards compressed so as to jiresent a plane surface. This lap, employed with ochre or 
rouge, enables the 023ticians’ method of polishing glass surfaces to be closely followed 
in the polishing of hard crystal surfaces. The second is a lap whose grinding surface 
is formed by a sheet of emery cloth stretched over, and cemented to, a metal base of 
the size and shape common to all tire laps. This lap has been particularly useful for 
elfecting the preliminary grinding down of the relatively large crystals employed in 
the work whose results are now being communicated, leaving but little for the 
ground-glass lap to do. The variety of eleven laps now provided, enables any or all 
of the usual grinding and polishing processes of the o})tician and lapidary to be 
followed, besides those described by the author for the grinding and polishing of the 
softer crystals of artificial salts. One of the laps is shown in position in the illustra¬ 
tion, and another to the left leaning against the base of the instrument. 
A further small but imjjortant addition to the accessories consists of three gripping 
crystal-holders, which are shown resting on the base. One of them is a triply and 
widely split tube of a centimetre bore, narrowing at a centimetre from the orifice into 
a cone which passes into a grooved stem similar to the stems of the ordinary holders 
used for wax attachment of the crystal. The wide splits are continued down to the 
stem, and the conical portion is })rovided A\ith a screw thread, Avith Avhich gears a 
