ntrolled by means of a telegraph-key and
the code may be rapidly transmitted. The search-light affords a means
for signaling great distances, even in the daytime. The light is usually
mechanically occulted by a quick-acting shutter, but recently another
system has been devised. In the latter the light itself is controlled by
means of an electrical shunt across the arc. In this manner the light is
dimmed by shunting most of the current, thereby producing the same
effect as actually eclipsing the light with a mechanical shutter. By
means of the search-light signals are usually visible as far as the
limitations of the earth's curvature will permit. By directing the beam
against a cloud, signals have been observed at a distance of one hundred
miles from the search-light despite intervening elevated land or the
curvature of the ocean's surface. By means of small search-lights it is
easy to send signals ten miles.

This kind of apparatus has the advantage of being selective; that is,
the signals are not visible to persons a few degrees from the direction
of the beam. One of the most recent developments has been a special
tungsten filament in a gas-filled bulb placed at the focus of a small
parabolic mirror. The beam is directed by means of sights and the
flashes are obtained by interrupting the current by means of a
trigger-switch. The filament is so sensitive that signals may be sent
faster than the physiological process of vision will record. With the
advent of wireless telegraphy light-signaling for long distances was
temporarily eclipsed, but during the recent war it was revived and much
development work was prosecuted.

The Ardois system consists of four lamps mounted in a vertical line as
high as possible. Each lamp is double, containing a red and a white
light, and these lights are controlled from a keyboard. A red light
indicates a dot in the Morse code and a white light indicates a dash.
The keys are numbered and lettered, so that the system may be operated
by any one. Various other systems employing colored lights have been
used, but they are necessarily short-range signals. Another example is
the semaphore. When used at night, tungsten lamps in reflectors indicate
the positions of the arms. The advantage of these signals over the
flashing-system is that each signal is complete and easy to follow. The
flashing-system is progressive and must be carefully followed in order
to obtain the meaning of the dots and dashes