Optical telegraphy, by reason of its very principle, presents both the
advantage and inconvenience of leaving no automatic trace of the
correspondence that it transmits. The advantage is very evident in
cases in which an optical station falls into the hands of the enemy;
on the other hand, the inconvenience is shown in cases where a badly
transmitted or badly collated telegram allows an ambiguity to stand
subject to dispute. Moreover, in case of warfare between civilized
nations that have all the resources of science at their disposal,
there may be reason to fear lest one of the enemy's optical stations
substitute itself for the corresponding station, and take advantage of
the situation to throw confusion into the orders transmitted. The
remedy for this appears to reside in the use of cryptography and in
the exchange, at various intervals, of certain words that have been
agreed upon beforehand, and that the enemy is ignorant of.
As for the automatic preservation of telegrams, the problem has not
been satisfactorily solved. It has been proposed to connect the key of
the manipulator of the optical apparatus with the manipulator of an
ordinary Morse apparatus, thus permitting the telegram to be preserved
upon a band of paper. It is unnecessary to say that the space occupied
by a dispatch thus transmitted would be considerable; but this is not
what has stopped innovators. The principal objection resides in the
increase in muscular work imposed by this arrangement upon the
telegrapher. Obliged to keep his eye fixed intently at the receiving
telescope, while at the same time maneuvering the manipulator and
spelling aloud the words that he is receiving, the operator should
have a very sensitive manipulator at his disposal, and not be
submitted to mental or physical overtaxation. So the apparatus that
have been devised have not met with much success.
Two French officers, working independently, have hit upon the same
idea of receiving the indications transmitted by the vibration of the
luminous fascicle directly upon their travel. The method consists in
the use of that peculiar property of selenium of becoming a good
conductor under the action of a luminous ray, while in darkness it
totally prevents the passage of the electric current. Such
modification of the physical properties of selenium, moreover, occurs
without the perceptible development of any mechanical work. If, then,
in the line of travel of the luminous fascicle emitted by the optical
apparatus, or in a portion of such fascicle, we interpose a fragment
of selenium connected with the two poles of a local pile, it is easy
to see that the current from the latter will be opened or closed
according as the luminous ray from the apparatus will or will not
strike the selenium, and that the length of time during which the
current passes will depend upon the length of the luminous attacks. A
Morse apparatus interposed in this annexed circuit will therefore give
an automatic inscription of the correspondence exchanged. Such is the
principle. But, practically, very great difficulties present
themselves, these being connected with the rapid weakening of the
electric properties of the selenium, and with the necessity of having
recourse to infinitely small mechanical actions only. The problem is
nevertheless before us, and it is to be hoped that the perseverance of
the scientists who are at work upon it will some day succeed in
solving it.
Finally, we may call attention to the attempts made to receive the
luminous impression upon a band prepared with gelatino-bromide of
silver. In practice this band would unwind uniformly at the focus of
the receiving telescope, which would be placed in a box, forming a
camera obscura. The velocity of this band prepared for photographing
the signals would be regulated by clockwork. The experiments that have
been made have not given results that are absolutely satisfactory, by
reason of the length of the signals received and the mechanical
complication of the device.