Vol. 7, 1921 
PHYSICS: C. BARUS 
73 
head h of the screw is graduated. The barrel b is at right angles to the 
tube aa', which at a joins the capillary tube d, leading to t of figure 1. 
At the end a' there is a cock, C, which shuts off communication with the 
atmosphere. Thus when C is closed pressure is applied directly at A, 
figure 1, by rotating the head h in figure 2. This pressure is at once re- 
moved by opening C. 
The apparatus worked surprisingly well. When C is closed and h 
rotated, the fringes may be placed anywhere in the field about as con- 
veniently as with the micrometer screw at the mirror of the interferometer. 
There is, however, one difficulty which I have not thus far been able to 
remove. When the pressure increments exceed a certain small value, 
the plates gg' no longer rise and fall in parallel. The coincidence of images 
is destroyed and the fringes vanish. There is here a conflict with the capil- 
lary forces present at the edges of the disc. I endeavored to improve this 
by using small plates gg f , anchored near the centre of MM' by 4 loose 
threads. But the advantage was not marked. Fringes a scale part in 
size will not be available for more than 50 scale parts, being sharpest in 
the middle. This is about half the diameter of field of the ordinary tele- 
scope. Curiously enough, fringes from the free surfaces of mercury 
vanish in the same way, probably owing to surface viscosity of the mer- 
cury, not absolutely pure. 
3. Equations and Pressure Observations. — If the cock C, figure 2, is 
closed and the temperature for brief intervals is considered constant, 
Boyle's law may be written (ignoring signs of increments) 
dv dV _ dp _ dh . . 
V ~ V " J ~ 76 ( } 
where V is the total volume enclosed, dv the increment at the micrometer 
screw hs, and dV the corresponding decrement equivalent to the pressure 
decrement dp. If a is the area of the piston at g dV = a dh/2 and if 
V — aH, H being the corrected air space at A, equation (1) becomes 
dv dh _ dh (C} , 
V ~ 2H ~~ 76 U 
But dh on the interferometer is equivalent to n fringes of wave-length X 
so that dh = nK/2. Hence, finally 
* = y\(k + (3) 
This equation gives a test of the trustworthiness of the gauge. 
In the apparatus used the following constants were found by measure- 
ment: V = 66.8 cm. 3 , a = 29.2 cm. 2 , H = 2.29 cm. The pitch of the 
screw was 0.073 cm. and its mean diameter 0.51 cm. Hence per turn 
dv = 0.073 X 0.204 = 0.0149 cm. 3 and dv/V 10~ 4 X 2.23 per turn. 
The mean wave-length being X = 6 X 10 ~ 5 cm. equation 3 reduces to 
n = 3.2 fringes per turn of the screw hs. 
