NEWTONIAN CONSTANT. OF GRAVITATION. 33 
of the mirror of °7”, and of the balls of zsglpqq inch. To this degree of accuracy 
there is no difficulty in observing; in fact, I might have made the divisions smaller, 
and still have been able to read to tenths. Of course this perfection is only possible 
with a very perfect mirror, and for this I applied to Mr. Hincer, who took special 
pains in preparing several as thin as he dare. Of these, one only, when tested while 
still round with the large astronomical telescope upon an artificial star, was perfect 
in its definition, and formed the spurious disc and diffraction rings equally in all 
directions. ‘This mirror was the one employed in all the experiments up to date. A 
second one was nearly as good. One made of quartz showed the diffraction rings 
strongly in three directions only, 120° apart. In making the test I was careful to 
notice in which direction the two images, one due to the front surface and the other 
to the silvered back surface, were separated, owing to the inevitable want of perfect 
parallelism between the faces. I then cut them in such a direction that the displace- 
ment should be vertical, so as to avoid the confusion caused by the superposition of 
the dim reflection of the scale upon the image under observation. The two good 
mirrors I slit with a fine steel disc and diamond dust, so as to leave a central bar 
4 inch wide. This gives abundant light, and defines well enough to enable the 
figures tobe read. I found by the use of screens that a bar $ inch wide, though it gave 
enough light, so destroyed the vertical definition that the figures could not be read, 
and the long and short lines were not so clearly distinguished. I then, with a very 
sharp-edged brass dise and washed emery, ground in the thickness of each mirror at 
each end a vertical V, seen in the plan (fig. 7). The bottom was so fine and sharp 
that a quartz fibre 7,455 inch in diameter would rest definitely in its place. The 
mirror was cemented with three spots of shellac varnish to the gilt copper support O. 
This was so formed that the balls could hang by their eye-hooks from the notches at the 
end of the arms, with the fibres resting in the vertical V grooves of the mirror. The 
beam mirror was carried by a quartz fibre, fastened to the point of O by shellac in all 
experiments up to No. 3, and soldered to an intermediate tag in the later ones. The 
details of the soldering process are given in the ‘ Phil. Mag.’ for May, 1894. From the 
lower central hook of the beam mirror the silver counterweight K may be suspended 
when the gold balls are removed. This was turned by Mr. CoLEBrook out of pure 
silver, which I had prepared by casting and hammering. It is truly cylindrical, and, 
with the triangular wire hook to which it is soldered, weighs exactly the same as the 
corresponding pair of balls with their fibres and hooks. Its diameter was measured 
in several places with a screw micrometer. The weight of the cylinder, of the little 
hook, and of the solder used were separately determined, and the very small radius of 
gyration of the hook esiimated from its dimensions and form. The period of the mirror 
was observed (a) with the balls on, (b) with the counterweight on, and (c) alone. From 
(a) and (b) the unknown moment of inertia of the beam could be determined, and 
from a, b, and c the effect of the stretching upon the rigidity of the fibre could be 
ascertained. The suppositions made are rather numerous, and are best discussed here, 
MDCCCXCV. —A, F 
