A NEW INTERFERENTIAL FORM OF ELASTICITY APPARATUS. 151 
level with the tops of the interference tripod screws when the latter are screwed fairly 
deeply into their columnar nuts. The black-glass surface can be adjusted absolutely 
horizontal by the means which have been indicated, and with the aid of a miniature 
spirit level provided; it is then immediately capable of receiving the rays of light 
directed down the vertical tube of the optical apparatus, and of reflecting them back 
again along the same path. The tripod screws can then be adjusted to support the 
colourless glass disc, so that its lower surface is separated from the black glass by the 
necessaiy thickness of air-film to allow for the further approach, without touching, of 
the hlack to the colourless glass, wdiich accompanies the bending of the plate. The 
further fine adjustment required for the production of suitable interference bands, and 
the measurement of their position with respect to the reference centre of the silver 
ring, are carried out precisely as described in the dilatometer memoir (p. 339). 
Mechanical “ Fingers"' for Adjusting the Plate. 
These aie carried by a fitting to the left side of the steel block. The immediate 
suppoits of the i^late are two gun-metal knife-edges carried on a pair of miniature 
spring-tables, provided with adjustments for height, mutual seiDaration, and for 
movement in an out of the recess between the blocks. The fitting consists of a thick 
plate screwed to the iron lilock, carrying as part of the same casting a thick-walled 
cylinder. Within this slides a shaft, on the lower part of which a screw thread is cut, 
and which is made capable of vertical motion only by the usual slot-and-pin device. 
The upward or downward motion is effected by rotation of a horizontal bevel wheel, 
situated near the base of the cylinder and provided witliin Avith a screw thread 
gearing with that of the shaft, so that ft acts as a driving nut. This gears with a 
smaller vertical one mounted at the end of a long axle terminating Avithin easy reach 
of the observer in a milled head. This axle is supported in two bearings, about whicli 
it is flanged, one forming part of the main casting of the fitting and the other iieiiig 
fixed to the iron block near the front end. 
The vertical shaft carries a stout head-piece, Avhich terminates above in a giooAmd 
bed, for the sliding horizontal in-and-out motion parallel to the front edge of the 
basal slab; the slider carries a rack beneath, gearing with a strong pinion carried in a 
hollow of the head-piece and manipulated by a milled head at the front end of a 
somewhat long axle. The slider in turn carries another horizontal grooved bed at 
light angles to the first. There are two sliders on this bed, each carrying one of the 
little supporting tables for the plate under investigation. They are arranged to move 
equally from or to the centre by means of a right and left-handed screw, carried in 
the bed and capable of rotation only, and gearing with corresponding nuts fastened 
beneath the sliders in a way which permits of some adjustment of tlie centre. 1’he 
screAv terminates in front in a milled head, just above that which effects the in-and- 
out motion. Each slider carries, flush Avith and in the middle of its inner edne a 
cTi * 
