There are two ways of solving the difficulty: first, by using a lens of
great focal length; and secondly, by placing the revolving mirror within
the principal focus of the lens. Both means were employed. The focal
length of the lens was 150 feet, and the mirror was placed about 15 feet
within the principal focus. A limit is soon reached, however, for the
quantity of light received diminishes very rapidly as the revolving mirror
approaches the lens.




Arrangement and Description of Apparatus.



Site and Plan.


The site selected for the experiments was a clear, almost level, stretch
along the north sea-wall of the Naval Academy. A frame building was
erected at the western end of the line, a plan of which is represented in
Fig. 3.

[Illustration: FIG. 3.]

The building was 45 feet long and 14 feet wide, and raised so that the
line along which the light traveled was about 11 feet above the ground. A
heliostat at H reflected the sun's rays through the slit at S to the
revolving mirror R, thence through a hole in the shutter, through the