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.
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.
Figs. 21 and 22. - FRONT VIEW AND LONGITUDINAL SECTION OF THE MANGIN PROJECTOR. (Scale 1/15). A. Elliptical mirror. B. Arm of the same. C. Nut for fixing the mirror. D. Support of the mirror. E. Occultator. F. Support for same. G. Lever for maneuvering the occultator. I. Support of the occultator rod. J. Screw for fixing the mirror support. K. Screw for fixing the support of the occultator rod. L. Screw for fixing the occultator support.