or 300140 kilometers per second.

In the following July the sum of $2,000 was placed at my disposal by a
private gentleman for carrying out these experiments on a large scale.
Before ordering any of the instruments, however, it was necessary to find
whether or not it was practicable to use a large distance. With a distance
(between the revolving and the fixed mirror) of 500 feet, in the
preliminary experiments, the field of light in the eye-piece was somewhat
limited, and there was considerable indistinctness in the image, due to
atmospheric disturbances.

Accordingly, the same lens (39 feet focus) was employed, being placed,
together with the other pieces of apparatus, along the north sea-wall of
the Academy grounds, the distance being about 2,000 feet. The image of the
slit, at noon, was so confused as not to be recognizable, but toward
sunset it became clear and steady, and measurements were made of its
position, which agreed within one one-hundredth of a millimeter. It was
thus demonstrated that with this distance and a deflection of 100
millimeters this measurement could be made within the ten-thousandth part.

In order to obtain this deflection, it was sufficient to make the mirror
revolve 250 times per second and to use a "radius" of about 30 feet. In
order to use this large radius (distance from slit to revolving mirror),
it was necessary that the mirror should be large and optically true; also,
that the lens should be large and of great focal length. Accordingly the
mirror was made 1¼ inches in diameter, and a new lens, 8 inches in
diameter, with a focal length of 150 feet was procured.

In January, 1879, an observation was taken, using the old lens, the mirror
making 128 turns per second. The deflection was about 43 millimeters. The