APPARATUS FOR ASCERTAINING MINUTE STRAINS. 91 
Big, 1. 
side, each carrying a telescope ¢ with adjusting appliances and a 
scale i which is divided into millimeters. The scale is seen clearly 
in the telescope reflected by the mirror, and as the mirror rotates 
slowly in consequence of the elongation of the test pieces, the 
image of the scale moves in the focus of the telescope and defines 
the tangents of the double angle through which the prisms, and 
Consequently the mirror, has revolved. The proportion between 
the elongation of the specimen and the reading of the scales is 
determined as follows :— 
Let r denote the width of the prisms, and # the distance 
between the scales and the mirror. Then if U be the elongation 
we have approximately— 
lagy 
The mean width of the prisms in the apparatus shown is 4°5402 
millimeters, and 2 is made 1135 millimeters 
1 
v= 500 
Now since differences of 4; of a millimeter can be easily defined 
on the scale, the extension corresponding with this reading is sv'so 
te millimeter, and the total of both readings with rodoo of @ 
millimeter, so that this apparatus is capable of showing elongations 
