150 
DR. A. E. H. TUTTON ON . THE ELASMOHETER, 
the best of six which had been prepared with the greatest possible care, the test 
being the production of rectilinear and regular interference bands when used, as in 
the determinations, in co-operation with the silver-ring surface of the large colourless 
glass disc. The selected disc is cemented by its ground under surface to the some¬ 
what thicker disc of brass. The third metallic disc is rigidly screwed at its centre on 
to the end of the vertical rod, and upon it the thicker brass disc with its attached 
glass one is mounted in a manner which permits of adjustment, by means of four 
capstan-headed screws arranged at equidistant intervals from each other near the 
circumference. Two 22 are situated along the diameter parallel to the horizontal 
stem of the rocker, and are adjusting screws, screwing only through the third fixed 
disc and merely pressing upwards against the middle disc ; the other pair z', along 
the diameter at right angles, are fixing screws, and pass loosely through the bottom 
disc, but screw into the middle one. The hlack-giass surface can thus be adjusted by 
the screws 22 about that diameter as axis which is perpendicular to the plane of the 
T-piece. Fine adjustment about the diameter parallel to the stem is afforded b}" the 
two fixing screws 2 ', and is roughly provided for in the mode of attachment of the 
agate wedge y'. This latter tapers to a knife-edge 2’1 centims. long, from which the 
greater portion of the actual edge about the centre has been hollowed awav, leaving 
only the two end parts with which to rest on the agate plate v. The wedge is 
mounted in a metallic holder carrying above its centre a split collar, through which 
tlie horizontal stem of the T-piece readily slides ; a tightening capstan screw, passing- 
through flanges standing up from the split edges, enables it to immovably grip the 
stem when the agate wedge has been adjusted to its proper position. 
Between the agate wedge and the free end of the stem a short cylindrical counter¬ 
poise x' is arranged, which is adjustable to the required position for nicely balancing 
the weight on the other side of the wedsre. The stem slides somewhat tio-htlv 
O O «. 
through the counterpoise on account of a bent spring let into a groove cut along its 
bore, which serves also the purpose of automatically fixing the counterpoise on the 
stem after adjustment. 
When in position, it will be obvious that the transmitter rests in stable equilibrium, 
in accordance with three-point contact, with the point of its vertical rod on the centre 
of the plate under Investigation and the two end-points of the agate wedge on the 
agate plane. The position of the latter can be regulated along its guiding bed so as to 
suit the particular separation of the blocks, as determined by the size of plate avail¬ 
able, so that the wedge shall lie symmetrically along its middle line. The counterpoise 
can be adjusted so as to regulate the weight with which the platinum-iridium point 
presses on the jilate of the substance down to zero, which, however, it is advisable to 
stoj^ just short of. The stem lies approximately horizontal and passes with ample 
room between the two screws of the interference tripod carried by the slider on the 
front block. The vertical rod passes readily lietween the two inner overhanging ends 
of the upper block, and the black-glass surface at its head is at a convenient height about 
