[Illustration: FIG. 23.]
The projectors employed for lighting to a distance the surroundings of
a stronghold or of a ship have likewise been applied in optical
telegraphy. For this purpose Messrs. Sautter, Lemonnier & Co. have
added to their usual projecting apparatus some peculiar arrangements
that permit of occultations of the luminous focus at proper intervals.
Figs. 21 and 22 show the arrangement of the apparatus, the principle
of which is as follows: When the axis of the projector points toward
the clouds, and in the direction occupied by a corresponding station,
the occultations of the luminous source placed in the focus of the
apparatus produce upon the clouds, which act as a screen, an alternate
series of flashes and extinctions. It is therefore possible with this
arrangement, and by the use of the Morse alphabet, to establish an
optical communication at a distance. The use of this projector (the
principal inconvenience of which is that it requires a clouded sky)
even permits two observers who are hidden from each other by the
nature of the ground to easily communicate at a distance of 36 or 48
miles.