A NEW INTERFERENTIAL FORM OF ELASTICITY APPARATUS. 
147 
facing front and the long edges parallel to the sides of the slab; it is 17 centims. 
high, 25 centims. long, and 9‘5 centims. wide, except at the base, where it is broadened to 
14 centims., in order to furnish a flange on each side, through which it is screwed down 
to the slab. This block carries above it, and sliding in a dove-tailed bed, two gun- 
metal blocks hh, intended to support the plate of the substance under investigation, c. 
The latter is not laid upon the edges of the bhwks, as in Koch’s apparatus, but is 
pressed upwards from below, by a point cl carried at the end of the balance beam c, 
against two platinnm-iridium knife-edges ff, arranged edge downwards in the roof of 
recesses in the blocks. 
The amount of movement of the centre of the plate, induced l)y the bending weight, 
is observed from above instead of from below, whicli latter is the method employed 
by Koch. The optical arrangements of the author’s interferometer are consecpiently 
directly applicable, and the totally-reflecting prism carried Ijy the Koch apparatus in 
the space between the blocks under the plate is eliminated. The plate is brought up 
to the knife-edges, and also prevented from falling out of position, when the upward 
pressure of tlie Ijending weight is removed, by a delicately adjustable pair of spring 
supports 'p'p'q, which gently press the plate up against the knife-edges without 
bending it. 
The two sliding blocks hh are 8 centims. long by 6‘2 centims. broad and high, and each 
is widened at the base into a dove-tail to fit the corresponding guides of the bed. For 
the purpose of ensuring adequately tight sliding, the left guide is furnished in each 
case by a stout bevelled rod of gun-metal, let in l^etween an upright flange of the steel 
block and the dove-tail of the sliding block. Each is fixed by two screws to the steel 
block, just sufliciently tightly to permit of steady travelling; a third screw, with 
capstan head and lever handle, is placed mid-way between them, and acts both as an 
adjusting screw wherewith to modify the tightness of the sliding, and as a clamping 
screw to fix the blocks in any desired position. The motion of the two l^locks is 
arranged to be equal and opposite on each side of the centre, by means of a long 
shaft cj passing through both, having a left-handed screw cut in its hinder portion 
and a right-handed one in the portion near tlie front, where it is rotatable in a stout 
hearing h fixed to the end of the iron block. It terminates in a large milled head i, 
rotation of which, in the direction of the hands of a watch, causes the blocks to recede 
from each other, while rotation in the opjDOsite direction brings them nearer together. 
The upper paiTs kk of the blocks which carry the platinum-iridium knife-edges J^' 
are separate castings fixed on the top of these sliding parts, and are 5'7 centims. high and 
2T centims. wide, except at the base, where they are widened to 4’4 centims., to permit 
of screwing down to the sliding blocks. This is done rigidly by four screws in the case 
of the hinder block; when they are screwed home the platinum-iridium knife-edge is 
truly parallel to the basal slab. In the case of the front block, means of adjustment 
for both azimuth and altitude are provided, to enable the platinum-iridium knife-edge 
which this block carries to be set absolutely parallel to that carried by the hinder 
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