A NEW INTEIiFEEENTIAL FORM OF ELASTICITY APPARATUS. 
14!) 
of the fellow wedge. There are three advantages offered by this form, namely, the 
knife-edge is less likely to cut into a relatively soft substance, a smaller plate of the 
latter can be employed, and the whole of each edge can be seen in the reading 
microscope. 
The Interference Tripod. 
At the top of each block, at the inner overhanging end, a guiding bed r is fixed for 
a sliding table s slightly over 3 centims. square, but with the inner corners cut off, so 
as not to impede the view through the microscope of the platinum-iridium points. 
The slider on the back block carries, near the middle of its inner edge, a single 
vertical screw t of hard white metal and very fine thread, and furnished with a 
milled head a few millimetres below its rounded summit, by means of which it can 
be raised or lowered in the brass columnar nut fixed to the slider. The slider on the 
front block carries two such screws, arranged also close to the inner edge and with 
their axes 2 centims. apart. These three screws form a tripod for tlie support of the 
colourless glass disc u, 4 centims. in diameter, whose lower surface, which has a 
minute silvered ring at its centre, is to assist in reflecting the interfering light. This 
disc is shown in the foreground in fig. 2, to the right, leaning against the edge of the 
iron base. The two sliders can be fixed in the positions found nn^st convenient for 
supporting tlie disc, by means of two hoi izontal clamping screws at their left sides. 
The Trcmsmitter. 
The front block also carries on the top of its front end a sliding agate table v, for 
the support of the agate wedge of the transmitter. The bevelled agate plate is 
capable of being accurately levelled by being fitted into a dove-tailed gun-metal 
setting, which is attached to the sliding table by three levelling and four fixing 
screws; the guides are grooves cut out of each side of the block, for a length parallel 
to the top edges of 4 centims. The slider can be fixed in the desired position by 
means of an oblique clamping sci'ew carried on the left side. 
The transmitter consists of a T-piece arranged on its side. It is shown in the 
foreground, to the left, in fig. 2, resting on the base. Its horizontal stem w 
is a rigid gun-metal rod of circular section and 17 centims. long, which joins 
perpendicularly the shorter upright cross-piece x, 6“2 centims. in length, at a point 
nearer to the upper than to the lower end. The latter terminates in a platinum- 
iridium point, intended to rest upon the centre of the plate c under investigation, 
while an agate wedge carried near its middle by the horizontal stem rests on the 
agate table. At the upper end the vertical rod carries a cylindrical head-piece, 
1'5 centim. in diameter and 1 centim. deep, Avhich is made up of three discs, the 
middle one being slightly thicker than the outer ones, and the parallel surfaces of 
which are approximately horizontal. The uppermost disc y is of black glass; its 
upper surface furnishes the second of the interfering reflections, and has been ground 
and jjolished an optically true plane. The particular disc employed was chosen as 
