Optical communications by signals, during day and night, with
experienced men, may, in the absence of telephones, telegraphs, and
messengers, render important service when the distance involved is
greater than two thousand feet.
This mode of correspondence is based upon the use of the Morse
alphabet. The signals are divided into night and day ones. The day
signals are made with small flags. When these are wanting, sheets of
white cardboard may be used. The night signals are made with a lantern
provided with a support, which may be fixed to a wall or upon a
bayonet.
In day signaling, the dashes of the Morse alphabet are formed by means
of two flags (Fig. 23) held simultaneously at arm's length by the
signaler. The dots are formed with a single flag held in the right
hand (Fig. 24). In this way it is possible, by extremely simple
combinations, to establish a correspondence, and produce any
conventional signal. By means of relay stations, the signals may be
transmitted from one to another to a great distance.
In signaling with the lantern, long and short interruptions of the
luminous source are produced by means of a screen.