1904 - 5 .] Apparatus for Measuring Lateral Strains. 45S 
with lock nuts, and the pressure of the screw points on the 
specimen is regulated by a spring threaded over a hollow spindle 
E pivoted to one arm, this spindle being guided by a second, F , 
pivoted to the other arm : the compression of the spring is 
regulated by a nut G upon the outer spindle. 
The free ends of the tubes are prolonged beyond the screw 
grips, and one of them is fitted with an ebony finger H , having 
a thin steel plate 1 secured to its outer end, which presses against 
a double knife-edge J, seated in a shallow V-notch cut in the end. 
of the other arm. 
This knife-edge carries a mirror K pivoted upon a vertical 
spindle, and capable of adjustment about an horizontal axis also. 
An adjusting screw A, secured in one of the collars, bears against 
the specimen, and keeps the instrument from swinging round on 
the points of the screws. 
With this arrangement any alteration in the diameter of the 
specimen between the screw points causes a movement of the 
outer end of one arm relatively to the other, and a proportional 
rotation of the knife-edge and its attached mirror is obtained. 
This rotation is observed by a telescope and scale placed at a 
convenient distance away, and a measure of the change in the 
