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. OPTICAL TELEGRAPHY BY MEANS OF PROJECTORS. [Illustration: FIG. 23.]