uide the mariner, except that the traveler
in the aerial ocean is far above the plane on which the beacon rests.
For this reason the lenses are designed to send light generally upward.
In foreign countries several types of beacons for aerial navigation have
been in use. In one the light from the source is freely emitted in all
upward directions, but the light normally emitted into the lower
hemisphere is turned upward by means of prisms. In a more elaborate
type, belts of lenses are arranged so as to send light in all directions
above the horizontal plane. A flashing apparatus is used to designate
the locality by the number or character of the flashes. Electric
filaments and acetylene flames have been used as the light-sources for
this purpose. In another type the light is concentrated in one azimuth
and the whole beacon is revolved. Portable beacons employing gas were
used during the war on some of the flying-fields near the battle front.

All kinds of lighting and lighting-devices were used depending upon the
needs and material available. Even self-luminous paint was used for
various purposes at the front, as well as for illuminating watch-dials
and the scales of instruments. Wooden buttons two or three inches in
diameter covered with self-luminous paint could be fixed wherever
desired and thus serve as landmarks. They are visible only at short
distances and the feebleness of their light made them particularly
valuable for various purposes at the battle front. They could be used in
the hand for giving optical signals at a short distance where silence
was essential. Self-luminous arrows and signs directed troops and trucks
at night and even stretcher-bearers have borne self-luminous marks on
their backs in order to identify them to their friends.

Somewhat analogous to this application of luminous paint is the use of
blue light at night on battle-ships and other vessels in action or near
the enemy. Several years ago a Brazilian battle-ship built in this
country was equipped with a dual lighting-system. The extra one used
deep-blue light, which is very effective for eyes adapted to darkness or
to very low intensities of illumination and is a short-range light.
Owing to the low luminous intensity of the blue lights they do not carry
far; and furthermore, it is well established that blue light does not
penetrate as far through ordinary atmosphere as lights of other colors
of the same intensity.

The war has been respons