PHYSICS: C. BARUS 
329 
tried with telephones and the spectrum ellipses before ; l but these fringes do not 
easily admit of being drawn out into a ribbon and there is usually deficient 
light. 
2. Apparatus. — I began the work with two similar telephones, as shown at 
t, t', in figure 1. Small mirrors were rigidly attached to the center of the dia- 
phragms and each of the telephones secured on standards which admitted of 
adjustment around vertical and horizontal axes. The intermittent current 
was supplied at a, b from a small induction coil with a rheostat in circuit. Four 
clamp screws at c, d, were available for putting the telephone bobbins in series 
or in parallel. The current of one could be versed at k. White light, L, from 
a collimator, was reflected or transmitted by the half silvered mirrors, M, M', 
N, N', of the interferometer and from m, m', on the telephones as indicated by 
the arrows. M' was on a micrometer with the screw on the direction, n. To 
facilitate the finding of the fringes one of the telephones should also be on a 
micrometer with the screw normal to m' . The fringes when found are observed 
by the vibration telescope at T. 
The vibration telescope is shown in vertical section in figure 2, with the 
ocular at E and the objective originally as e, the tube being supported on the 
standard, d, and clamp cc, admitting of raising and lowering and of slight rota- 
tion around the horizontal axis, b. The objective, A, has been removed and is 
now supported by a flat steel spring, s, in front of its former position. 
In order that the objective may vibrate parallel to the fringes and as these 
appear in all angles of altitude, the special vibratory system /, g, k, s, has been 
