694 
PHYSICS: C. BARUS 
bifilar suspension,^ the threads of which are attached at d, d! . The abut- 
ments were heavy cast iron bricks, 2 X 3.5 X 10 cubic inches in size, 
firmly bolted together and standing with screws on a smooth bed plate. 
Thus the apparatus (not including the interferometer) can be rotated 
around a vertical axis to enlarge the achromatic fringes and around a 
horizontal axis parallel to F to rotate them. This is necessary for the 
adjustment. 
4 
of 
a 
e' 
c4 
4a 
d 
i Z i 
h 4 5^ 
t 
C 
# 
20 40 6( 
r 
d 
cfl' 
Omitting details I may add that all measurements were made in 
terms of the displacement of the achromatic fringes heretofore described. 
If E is the traction modulus, / the elongation of each rod of length 
L, under the force P, and A a differential symbol, 
E = (A F / A) / (A I / L) 
If the distance apart of the rays, a, b, is 2R, and of the forces of the 
couple is 2R\ 
2 R A a = A N cost. 
