A NEW INTERFERENTIAL FORM OF ELASTICITY APPARATUS. 
159 
In actual practice the control screw is found to work even more admirably than the 
author anticipated. The bands pass witli the utmost steadiness, and their counting 
is extremely easy. 
The Measuring Microscope. 
The measuring microscope is mounted, on the left-hand part of tlie basal slab, on a 
stout column of circidar section, tapering to the summit and expanding at the base to 
a flange of 14 centims. diameter. The under side of the base is cut with a bevelled 
groove, by which the column slides over a correspondingly dove-tailed guiding bed of 
iron, 22’5 centims. long. A strong rack is longitudinally sunk in the bed, and gears 
with a jDinion borne within the lower part of the column, which has here a jjrojection 
coining out to the flange in front to afford a rigid axle-bearing and permit of the 
attachment of a winch handle. One half-turn of this handle causes the column to 
travel rapidly over the rack from left to right or vice versd, so as to bring the 
microscope over the blocks or remove it out of the way. It can be rigidly fixed in 
any position on its path by a very strong vertical Ijinding screw, furnished at its head 
with a lever handle, and screwing through the basal flange, at the back of the column ; 
the screw shaft terminates below in a conical head, which lies within a corresponding 
conical hole in a bevelled clamping slip inserted against the back guide. Rotation of 
the handle for half a revolution draws up the cone sufficiently to press the clamping 
slip tightly against the guiding bed, and so binds the column to the basal slab. 
The column is 22'5 centims. high, and carries on its expanded tabular top the two 
rectangular measuring movements. The guiding bed of the lower one, which is that 
at right angles to the front edge of the basal slab, aud all that it carries is rotatable 
for about 20° in the horizontal plane about the axis of the column, and capable of a 
considerable amount of fine adjustment and of eventual rigid fixation at any azimuth 
within those limits, by means of two opposite capstan-screws directed, thrcmgh the 
sides of a rectangular hollow bracket projecting from tbe back of a collar I'ound the 
tabular top, against a stud projectiiig into the hollow from below the guiding bed. 
This enables the upper right-and-left measuring movement to be set exactly parallel 
to the fixed platinum-iridium knife-edge AB carried by the hinder l)lock. The 
thick slider, 8'5 centims. square, which moves over the lower guiding bed carries the 
guiding bed of this second movement, fixed exactly at 90° to the lower one. The 
similar slider which moves over this bed carries a vertical column 20 centims. 
high, from an arm of which the microscope is vertically STispended. The 
column consists of a fixed conical core and a rotataljle outer part, so that the 
microscope can be swung readily out of tlie way of the interferometer. The outer 
rotatable part of the column can be fixed in aiiy position about the core, by means of 
a washer, with hole to fit a squared part of the core, which projects above the 
rotatable column, followed by a large milled-headed clamping nut which screws on a 
thread with which the core terminates. 
