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.

Smaller signal-lamps using acetylene have been employed in the forestry service and in other activities where a portable device is necessary.  In one type, a mixture-tank containing calcium carbide and water is of sufficient capacity for three hours of signaling.  A small pilot-light is permitted to burn constantly and the flashes are obtained by operating a key which increases the gas-pressure.  The light flares as long as the key is depressed.  The range of this apparatus is from ten to twenty miles.  An electric lamp supplied from a storage battery has been designed for geodetic operations in mountainous districts where it is desired to send signals as far as one hundred miles.  Tests show that this device is a hundred and fifty times more powerful than the ordinary acetylene signal-lamp, and it is thought that with this new electric lamp haze and smoke will seldom prevent observations.

Certain fixed lights are required by law on a vessel at night.  When it is under way there must be a white light at the masthead, a starboard green light, a port red light, a white range-light, and a white light at the stern.  The masthead light is designed to emit light through a horizontal arc of twenty points of the compass, ten on each side of dead ahead.  This light must be visible at a distance of five miles.  The port and starboard lights operate through a horizontal arc of twenty points of the compass, the middle of