and the dielectric between them was a cube cut
from a crystal, so that two of its faces should be perpendicular to the
optical axis, whilst the other four were parallel to it. A small projecting
piece of shell-lac was fixed on the inductric ball at that part opposite to
the attachment of the brass rod, for the purpose of preventing actual
contact between the ball and the crystal cube. A coat of shell-lac was also
attached to that side of the carrier ball which was to be towards the cube,
being also that side which was furthest from the repelled ball in the
electrometer when placed in its position in that instrument. The cube was
covered with a thin coat of shell-lac dissolved in alcohol, to prevent the
deposition of damp upon its surface from the air. It was supported upon a
small table of shell-lac fixed on the top of a stem of the same substance,
the latter being of sufficient strength to sustain the cube, and yet
flexible enough from its length to act as a spring, and allow the cube to
bear, when in its place, against the shell-lac on the inductric ball.

[Illustration:]

1691. Thus it was easy to bring the inducteous ball always to the same
distance from the inductric bull, and to uninsulate and insulate it again
in its place; and then, after measuring the force in the electrometer
(1181.), to return it to its place opposite to the inductric ball for a
second observation. Or it was easy by revolving the stand which supported
the cube to bring four of its faces in succession towards the inductric
ball, and so observe the force when the lines of inductive action (1304.)
coincided with, or were transverse to, the direction of the optical axis of
the crystal. Generally from twenty to twenty-eight observations were made
in succession upon the four vertical faces of a cube, and then an average
expression of the inductive force was obtained, and compared with similar
averages obtained at other times, every precaution being taken to secure
accurate results.

1692. The first cube used was of _rock crystal_; it was 0.7 of an inch in
the side. It presented a remarkable and constant difference, the average of
not less than 197 observations, giving 100 for the specific inductive
capacity in the direction coinciding with the optical axis of the cube,
whilst 93.59 and 93.31 were the expressions for the two transverse
directions.

1693. But with a second cube of rock crystal corresponding results were not
obtained. It was 0.77