330 
PHYSICS: C. BARUS 
devised. The rod /, reaching downward (about 10-15 cm. long) is attached to 
a ring of the objective, rr, capable of revolving with friction around the tube 
of the objective and of being fixed for any position in altitude of the rod. The 
horizontal clamp, g, adjustably attached to the rod, /, supports the spring, s. 
This passes through a vertical crevice in g; it is fixed by the set screw, p, and 
the two oblique adjusting strips, k, on either side of g. These pieces, k, are 
clamped by the screws at h, and serve as holders of the two coaxial set screws 
at i, on either side of s, so that the objective A, may be centered relative to the 
tube, Ee, for any oblique position of /, which must be normal to the fringes. 
The vibration may frequently be considerably changed by sliding / and s in g 
and reclamping the system. If the objective is to be fixed, a screw at n may 
be depressed for the purpose. 
Under all circumstances the two spots of light representing the slit, if caught 
objectively on a screen at T, must be nearly coincident, horizontally and ver- 
tically, when the rays at T are parallel. If these spots are too far apart the 
fringes will be very small or even absent. 
3. Observations with telephones. — The use of two telephones soon showed it- 
self to be unsuitable for the present purposes, for the diaphragms oscillate not 
merely fore and aft (as in here desirable) but around horizontal and vertical 
axes as well, particularly when the vibration is relatively intense. Hence the 
coincident slit images of the silent telephone periodically separate. This 
shows itself in a peculiar manner in the field of the vibration telescope. The 
whitish field carrying the fringe waves due to the fore and aft motion of the 
mirror, m, ni', figure 1, on the diaphragm of the telephones, is intersected by 
nearly equidistant vivid white lines, normal to the waves. The latter are apt 
to be broken and appear only as traces between the vertical lines in question. 
To account for these occurrences of the lines, it is sufficient to recall that the 
originally coincident identical fine slit images are vibrating through each 
other in some direction effectively normal to the slit. Hence these images 
will be seen clearly only at the elongations, when the slit images are temporarily 
stationary and at a maximum distance apart. Hence also the waves are 
absent at the lines, for here the slit images are so far apart as to eliminate the 
interferences. 
As it made little difference how the telephones were electrically connected 
the diaphragms were removed and replaced by the two long strips of steel made 
from hack saw blades rigidly attached to the body of the telephone by a 
metallic arch. To approach this as near the magnet as possible forcing screws 
were provided at a little distance from the end of the strip. In this case the 
vibration of the strip and the mirror at its middle was fore and aft only and as 
a consequence the bright lines intersecting the field vanished completely. The 
arrangement of telephones, whether in series or parallel made a decided dif- 
ference in the amplitude of the waves, which could be increased many times the 
breadth between successive fringes before the waves became turbulent and 
broke up. 
