340 
PHYSICS: C. BARUS 
m, to be d = 6 cm., and M = 10 3 grams, if the rigidity of quartz is taken as 
5 x 10 11 , t = 6 X 10 6 r 4 AN. Thus if 
r = 10- 2 cm., AN = 1.1 X 10~ 6 cm. per kg. of M, Period T = 4.6 sec. 
r = 10-3 cm., AN = 1.1 X 10" 2 cm. per kg. of if, T= 465 sec. 
The first case would then correspond to but 1/40 of a fringe; in the second case 
there should be 250 fringes per attracting kilogram. The working conditions 
below proved to be much better than this, rigidity being estimated too high. 
If the tenacity of quartz be taken as 1.5 X 10 3 kg. per square centimeter 
the latter filament (r = 10~ 3 cm.) should still hold 4.5 grams, or much more than 
the weight of the above needle (within 2 grams). Having mounted the needle 
and found the fringes, the image of the wide slit (i.e., the reflected beam seen 
in the telescope) was apparently quite stationary, the light needle being thus 
adequately damped. But within this virtually stationary slit image, the 
fringes (preferably made horizontal) during the winter months continually 
wandered up and down, showing that micrometric vibration had not been 
eliminated. The experiment is a very impressive one; but the drift in a 
heated room is so large that all attempt at measurement is idle. 
In. the summer installation in a subcellar at nearly constant temperature 
with a few improvements of apparatus (the mirrors being readjusted, etc.) the 
needle was without difficulty made to take a stable position midway between 
the glass plates, subject to the torsion of the fiber. With the micrometer at 
45 degrees, the latter is limited in its displacement. I therefore later attached 
a special micrometer with three identical pairs of parallel V-mirrors (the latter 
at 90 degrees) as shown at n, n', n", in figure 4, M, M', N, N', being the in- 
terferometer mirrors, L the collimator and T the telescope. This design 
when the mirrors are reciprocally parallel, has the additional advantage of 
being independent of slight changes of inclination in the micrometer. The 
displacement due to V-mirrors is now virtually parallel to the rays and no 
difficulty in finding the fringes need occur. Naturally the mirrors must be 
good, there being now four additional reflections in each ray; and the V-ap- 
paratus must be accurately adjusted for parallelism of mirrors. The central 
mirrors, n' ', only are moved by the micrometer screw. 
With the fringes found there is now no difficulty in showing the attraction 
of gravitation. In fact in iron brick moved on a small truck near the shot 
at one end of the needle, gripped the balls m, m' very much like a magnet act- 
ing on the pole of a magnetic needle. Throughout the whole of the experi- 
ment the fringes were under the perfect control of the micrometer. 
A more systematic experiment was made by testing the attraction of a lead 
ball 5.43 cm. in diameter and weighing about M — 950 grams, for the shot m' 
(at the end of the needle) weighing m' = 0.61 grams. The kilo was moved on 
a circular track with stops to a distance of R = 4.24 cm. (between centers of 
balls) from the ball of the needle, alternately. The period of the air-damped 
needle can not have been less than 18 min. The case is then that of a forced 
