brightness of a few modern light-sources are of
interest. These are expressed in candles per square inch of projected
area; that is, if a small hole in a sheet of metal is placed next to the
light-source and the intensity of the light passing through this hole is
measured, the brightness of the hole is easily determined in candles per
square inch.

BRIGHTNESS OF LIGHT-SOURCES IN CANDLES PER SQUARE INCH

Kerosene flame 5 to 10
Acetylene 30 to 60
Gas-mantle 30 to 500
Tungsten filament (vacuum) lamp 750 to 1,200
Tungsten filament (gas-filled) lamp 3,500 to 18,000
Magnetite arc 4,000 to 6,000
Carbon arc for search-lights 80,000 to 90,000
Flame arc for search-lights 250,000 to 350,000
Sun (computed mean) about 1,000,000

As the reflector of a search-light is an exceedingly important factor in
obtaining high beam-intensities, considerable attention has been given
to it since the practicable electric arc appeared. The parabolic mirror
has the property of rendering parallel, or nearly so, the rays from a
light-source placed at its focus. If the mirror subtends a large angle
at the light-source, a greater amount of light is intercepted and
rendered parallel than in the case of smaller subtended angles; hence,
mirrors are large and of as short focus as practicable. Search-light
projectors direct from 30 to 60 per cent. of the available light into
the beam, but with lens systems the effective angle is so small that a
much smaller percentage is delivered in the beam. Mangin in 1874 made a
reflector of glass in which both outer and inner surfaces were spherical
but of different radii of curvature, so that the reflector was thicker
in the middle. This device was "silvered" on the outside and the
refraction in the glass, as the light passed through it to the mirror
and back again, corrected the spherical aberration of the mirrored
surface. These have been extensively used. Many combinations of curved
surfaces have been developed for special projection purposes, but the
parabolic mirror is still in favor for powerful search-lights. The tip
of the positive carbon is placed at its focus and the effective angle in
which light is intercepted by the mirror is generally about 125 degrees.
Within this angle is i