In conclusion, it may be said that optical telegraphy, which has only within a few years emerged from the domain of theory to enter that of practice, has taken a remarkable stride in the military art and in science. It is due to its processes that Col. Perrier has in recent years been enabled to carry out certain geodesic work that would have formerly been regarded as impracticable, notably the prolongation of the arc of the meridian between France and Spain. Very recently, an optical communication established between Mauritius and Reunion islands, to a distance of 129 miles, with 24 inch apparatus, proved that, in certain cases, the costly laying of a submarine cable may be replaced by the direct emission of a luminous ray.
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Mr. F. Von Faund-Szyll has devised an original system of submarine telegraph, which is based upon the well known property that selenium exhibits of modifying its resistance under the influence of luminous rays, and which he styles the Selen-Differenzialrecorder.
Contrary to what is found in the other systems hitherto employed, the Faund-Szyll system utilizes the cable current merely for starting the receiving apparatus, which are operated by means of strong local batteries. The result is that the mechanical work that devolves upon the line current, which is, as well known, very weak, is exceedingly reduced.
The system consists of two essential parts: (1) The receiver, properly so called. (2) The relay as well as the registering apparatus or differenzialrecorder. The receiver consists of a closed box, K, in the interior of which there is a very intense source of light whose rays escape by passing through apertures, a a’, in the front part (Fig. 1).
As a source of light, there may be conveniently employed an incandescent lamp, g, capable of giving an intense light, and arranged (as shown in Fig. 2) behind the side that contains the slits, a a’.
The starting apparatus consists of a small galvanometric helix, r, analogous to Thomson’s siphon recorder, which is suspended from a cocoon fiber and capable of moving in an extremely powerful magnetic field, N S. This helix carries, as may be seen in Figs. 1, 3 and 4, a prolongation, v, at its lower end whose form is that of a prism, and which is arranged in front of the partition of the box, K, in such a way that it exactly covers the two slits, a and a when the bobbin is at rest, and in this case prevents the luminous rays of the lamp, g, from escaping from the box. But, as soon as the current sent through the cable reaches the spirals of the bobbin, through the conductors, y y’, the sum of the elementary electrodynamic actions that arise causes the helix to revolve to the right or left, according to the polarity of the current, while at the same time the helix slightly approaches one or the other of the poles of the magnet. The prolongation, v, of the helix, being firmly united with the latter, follows it in its motion, and has the effect of permitting the luminous rays to escape through one or the other of the slits, a a’, so that the freeing of the luminous fascicle, if such an expression is allowable, is effected.