462 MK. A. E. TUTTOX ON TFIE THERMAL DEFORMATION OF THE CRYSTALLISED 
angle, usually 90°, to the upper one. The two cylindrical movements, however, onlv 
admit of 35° of rotation on either side of the vertical axis in each case ; this amount 
is ample for ordinary purposes, including the preparation of a 60° prism hv separatelv 
setting the direction of each required prism-face, by rotation of 30° from the plane 
perpendicular to tlie bisecting plane, the former of which planes can usually be as 
readily goniometrically adjusted w'lth reference to the existing crystal faces as the 
bisecting jdane itself In the exceptional cases referred to, of which the extremelv 
deliquescent cresium selenate is an excellent instance, it repays to render the 
adjusting mechanism more cumbrous in order to secure the prime object, and for this 
reason the new alternative adjusting apparatus is provided. 
It is exactly like the one provided for <,)i’dinary use as far down as the upper fixed 
cylindrical segment and its divided silver scale reading 35° on each side, which is 
suspended by a bracket from the lower disc of the centering arrangement. The latter 
is given in duplicate, one being always attached to the ordinary and one to the 
special adjusting apparatus, as it is more readily attached to, or detached from, the 
inner axis of the goniometer than the adjusting apparatus to or fi'oin the centering- 
disc. Sliding in and under the fixed segment, instead of the usual movable segment 
of the same size, is one of do\dde the size, that is of rather more than 150°. On one 
face this enlarged movable segment carries a silver index, to indicate the position 
with respect to the fixed graduated arc above it ; and on the other a silver arc 
p’raduated to 75° on each side of the centre. In a rabbetted bed on the under side 
o 
of the large segment slides the carrier of the lower adjusting segments, which are of the 
same kind as in the ordinary apparatus, arranged permanently at right angles to the 
two upper ones, the hoi-izontal circle of the ordinary a})paratus being omitted in order 
to avoid conqdexity. The sliding of the large segment about the rq)per fixed one is 
effected, for the 35° of its path on each side, by manipulation of the milled head of a 
tangent screw arrangement as in the ordinary apparatus. The lowest of the pair of 
segments arranged at right angles to these upper ones is also manipulated in its 
segmental bed foi- 35° each side by a similar tangent screw. But the sliding of 
the carrier of the tvm lower segments about the large segment is effected by hand, 
and fixation at any required position, with reference to the laige silver arc as indi¬ 
cated l)y an index on the carrier, can be l_)rought about Ijy a milled-headed screw-clamp 
on the opposite side to that on which are situated the manipulating screw of the 
lower segments and the index just referred to. 
The mode of using the apparatus is very simple. The crystal is attached, with the 
minimum of wax protruding at the sides, to the smallest of the special crystal-holders 
which are provided with azimuth adjustment, and with the plane which is desired to be 
the bisectrix of the 60°-prism arranged vertically as nearly parallel to the goniometrical 
axis as possilde and parallel to the lower tangent screw. The latter can be accurately 
attained l.)y use of tlie azimuth adjustment of the crystal-holder. The ])lane referred to 
is then exactly goniometrically adjusted Avith the aid of the two tangent screws, that 
