908 
MR. A. B. TUTTON ON" AN INSTRUMENT FOR GRINDING SECTION- 
optical elasticity can l3e aiTived at and recorded upon the spherical projection of the 
crystal. It is then only a matter of interpreting the spherical projection mechani¬ 
cally, and utilising the movements provided with the instrument so as to bring the 
median lines perpendicular to the grinding plane. 
Grinding of the Secoiul Surface Parallel to the First. 
Having thus ground the first surface of the plate, it now only remains to grind a 
second surface parallel to it. This may be done if desired with the aid of the apparatus 
supplied by Fuess, alluded to at the commencement of this communication. It can, 
however, be much more neatly and accurately achieved, and without the disagreeable 
noise made by the steel screws grating over the grinding plate, by use of the instru¬ 
ment now described, with the aid of a special crystal holder. 
The crystal is first detached from the holder upon which it has been fixed during the 
grinding of tlie first surface, by removing the wax around it with a penknife ; the 
hard-setting wax employed by opticians is very convenient, as a gentle pressure 
of the knife-blade under the crystal after removing the wax around its sides is 
generally sufficient to detach it intact and unsoiled by the wax. It is then cemented 
by its ground and polished surface to the centre of a, circular glass disc, half an inch 
in diameter, cut out of the thinnest variety of microscope 3-inch by 1-inch slips, and 
with neatly ground circumference. Micro cover-glasses are too thin, they are too 
easily fractured. It is best to have a gross of glass discs made at once, cut exactly 
to the same size with the same tool. The cement used will depend upon the nature 
of the crystal. If it is an anhydrous salt which will not be likely to be injured 
by being raised to 60°--70°, Canada balsam, which has previously been heated for some 
days to about that temperature so that it sets immediately upon cooling, may be 
employed. With care the same mounting material may be used with many sub¬ 
stances which contain water of crystallization, and the grinding of the second surface 
can consequently be immediately proceeded with. It is safe, however, to employ 
balsam or otlier cement dissolved in a quickly evaporating solvent, such as a con¬ 
centrated solution of hard balsam in benzene, so as to avoid all risk, either of strain or 
of decomposition, by raising the temperature. Any good liquid cement which has 
effectual binding properties, hardens in a night, and is without action on the crystal, 
will answer the purpose, and a slight brown colour is no detriment provided it does 
not stain, for the well-polished section is to be unmounted again before use for the 
measurement of the optic axial angle. The disc upon which the crystal is mounted, 
after hard setting of the cement, is placed in the receptacle for it in the special 
holder, which will now be described. 
It consists of two parts, which are shown in fig. 1 in front of the base, very 
slightly to the right. The upper portion, which is represented nearest the front 
and most to the right in the illustration, consists of a thick brass disc, 1 inch 
