PHYSICS: C. BARUS 
415 
This is the curve given (with double ordinates for distinction), in curve 
6, figure 2, and in figure 3. Besides this the datum a = — .6°, N = — .320 
cm was obtained. In figure 3 the mean rate is 
agreeing with the preceding as closely as may be expected. We may 
thus estimate AN = 27 X 10~^ of displacement at the micrometer at 
Hs per micro-radian of turn a at the mirror m, which amounts to a 
little less than one interference ring per microradian (about one-fifth 
second of arc) of turn. The theory will be given later. With regard 
to the application to the electrometer we may come to the following 
conclusion. A good instrument of the quadrant or similar type should 
give about a radian of deflection per volt, or a microradian per micro- 
volt. In the present interferometer the microradian is about equiva- 
lent to the passage of one interference fringe. Hence one fringe per 
microvolt is about the order of sensitiveness to be expected. 
1 This contribution is a note from a Report to the Carnegie Institution of Washington. 
2 Washington, Carnegie Inst., Pub., No. 229, 1915, 10 et seqj 
3 Ibid., 67 et seq.) 
* The return rays may also be projected in a screen near the objective of the properly 
focussed collimator and the sharp images put in contact. 
" If a is the refraction of Ught transmitted and 1 — a reflected, the fraction of the original 
light, L, reaching the telescope T will be 2a^{l — a)^. This is a maximum if a = |. Thus 
the illumination is reduced to |. 
AN 
= .465 cm/degree, or 26.6 cm/radian. 
FIG. 2 AND 3. 
PROCEEDINGS OF NAT. ACAD. OF SCI., VOL. 3, NO. 6 
