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IV. The Elasmometer^ a New Interferential Form of Elasticity Ajyxiratus. 
By A. E. H. Tutton, D.Sc., F.li.S. 
Received May 12,—Read May 14, 1903. 
Being desirous of extending the investigation of the physical characters of the 
crystals of isomorphous salts to the subject of their elasticity, the question arose as 
to the best form of apparatus to employ for the determination of the coefficient of 
elasticity. The most accurate form hitherto devised is that of Koch (‘ Ann, der 
Phys.,’ N.F. 1878, vol. 5, p. 251, and 1883, vol. 18, p. 325). It was subsequently 
employed in the researches of Beckenkampe (‘Zeitschr. fur Kryst.,’ 1885, vol. 10, 
p. 41, and 1887, vol. 12, p. 419), of Vater (‘Zeitschr. fiir Kryst.,’ 1886, vol. 11, 
p. 549), and of Niedmann (‘Zeitschr. flir Kryst.,’ 1888, vol. 13, p. 362). The amount 
of flexure of a thin plate of the crystal was determined by the interference method, 
sodium light being employed. The plate was laid on the edges of two sloping blocks, 
and a bending weight applied above its centre, through a point at the end of a hook 
or a stirrupped knife-edge. The lower surface of the plate, and the upjier surface of 
a totally-reflecting prism supported very closely beneath it, with only a thin film of 
air intervening, were the two surfaces the light reflected from which was caused to 
interfere and to produce bands. 
A complicated and somewhat cumbersoine arrangement for suspending the weight 
from the hook or stirrup, through a hole in the table on which the instrument was 
supported, and of regulating and graduating the application of the weight, formed an 
essential part of the apparatus. 
The great convenience and high accuracy of the optical part of the interference 
dilatometer, which the author has previously described (‘Phil. Trans.,’ A, 1898, 
vol. 191, p. 313), suggested the advantage of utilising it as interferometer for the 
measurement of the amount of flexure of the plate, and many other possible improve¬ 
ments on the apparatus of Koch also suggested themselves. Eventually the 
instrument now’ described was devised. It is applicable to solid substances in general, 
whether transparent or opaque, and whether polishable or not. It has been constructed 
by Messrs. Troughton and Simms, the makers of the dilatometer. 
A general view of the whole apparatus, including the interferometer, is given in 
fig. 1, as it appears during the progress of an actual determination. The elasticity 
apparatus is shown in fig. 2, wnth the three principal parts wmll detached in order to 
reveal them clearly. 
VOL. CCH.—A 349. 
26.10.03 
