A NEW INTERFERENTIAL FORM OF ELASTICITY APPARATUS. 
155 
platinum-iridium knife-edges. This, with the rack adjustment, enables the point to 
be adjusted with great exactitude under the centre of that portion of the plate of 
the substance investigated which lies between the knife-edges. The azimuth move¬ 
ment is effected by mounting the central column on a circular dove-tailed base, 
which rotates in an outer circular guiding bed correspondingly bevelled but not 
quite so thick, and screwed down to a square basal plinth sufficiently tightly to 
prevent play; a thumbscrew passing through this outer ring, above which it is 
flanged, into the square plinth enables the inner rotating base and the column which 
it carries to be rigidly clamped down to the plinth after the adjustment has been 
completed. The inner rotating disc carries above it on the right a stout plate, from 
which projects an arm, the end of which lies between a milled-headed screw and 
spring piston mounted in uprights from the square plinth. Rotation of the milled 
head one way or the other thus brings about the desired rotation of the column and 
all that it carries. A pair of spirit levels are carried along the left and back edges of 
the plinth. 
The height of the balance has been carefully arranged, so that when at rest in its 
supporting frame the pressure-point is at a level about a millimetre lower than the 
plane of the })latinum-iridium knife-edges. Hence, when a plate is in position under 
the latter, and the pressure-point is brought under its centre, there is a space of at least 
one or two-tenths of a millimetre left between the point and the under surface of the 
plate, supposing the latter about the usual thickness of something under a millimetre. 
Hence the supporting frame can be left in operation up to the last moment, and when 
the beam is eventually released it remains practically horizontal during the bending 
of the plate. 
There is one occasion, however, when the beam should be lowered by about a 
centimetre, namely, when a test is being made for any movement of the knife-edges 
under the blocks with respect to the interference tripod screws on the top of the 
blocks. In the form of apparatus now described this movement is practically ral, 
but it is well to have a means of confirming the fact by a direct determination, I]i 
Koch’s apparatus it amounts to as much as the equivalent to two interference l)ands, 
as the second reflecting surface is not supported by the blocks, but indeiDendently. 
For this purpose instead of a plate a block of glass, 1 centini. thick, has 1jeen 
provided, which may be assumed to he unhendahle, and any movement of the 
interference bands on allowing the weinht to act as usual must he due to movement 
of the parts of the blocks, from knife-edges to tops of screws. 
In order to provide for this determination the square plinth is separated from the 
rectangular basal box by a similar square plate of Ijrass 1 centim. thick, and tlie 
fixing screws are long enough to j^ass through both plates into the box top. When 
the determination in question is to be made, this insertion plate is removed, and the 
square plinth fixed directly to the box. The beam will then he at the right height 
lor the experiment. 
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