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By means of this apparatus in the survey, signals were rendered visible at distances as great as one hundred miles. Drummond proposed the use of this light-source in the important lighthouses at that time and foresaw many other applications. The lime-light eventually was extensively used as a light-signaling device. The heliograph, which utilizes the sun as a light-source, has been widely used as a light-signaling apparatus and Drummond perhaps was the first to utilize artificial light with it. The disadvantage of the heliograph is the undependability of the sun. With the adoption of artificial light, various optical devices have come into use.

Philip Colomb perhaps is deserving of the credit of initiating modern signaling by flashing a code. He began work on such a system in 1858 and as an officer in the British Navy worked hard to introduce it. Finally, in 1867, the British Navy adopted the flashing-system, in which a light-source is exposed and eclipsed in such a manner as to represent dots and dashes analogous to the Morse code. At first the rate of transmission of words was from seven to ten per minute. Recently much more sensitive apparatus is available, and with such devices the rate is limited only by the sluggishness of the visual process. This initial system was very successful in the British Navy and it was soon found that a fleet could be handled with ease and safety in darkness or in fog. Inasmuch as the “dot-and-dash” system requires only two elements, it may be transmitted by various means. A lantern may be swung in short and long arcs or dipped accordingly.

The blinker or pulsating light-signal consists of a single light-source mechanically occulted. It is controlled by means of a telegraph-key and the code may be rapidly transmitted. The search-light affords a means for signaling great distances, even in the daytime. The light is usually mechanically occulted by a quick-acting shutter, but recently another system has been devised. In the latter the light itself is controlled by means of an electrical shunt across the arc. In this manner the light is dimmed by shunting most of the current, thereby producing the same effect as actually eclipsing the light with a mechanical shutter. By means of the search-light signals are usually visible as far as the limitations of the earth’s curvature will permit. By directing the beam against a cloud, signals have been observed at a distance of one hundred miles from the search-light despite intervening elevated land or the curvature of the ocean’s surface. By means of small search-lights it is easy to send signals ten miles.

This kind of apparatus has the advantage of being selective; that is, the signals are not visible to persons a few degrees from the direction of the beam. One of the most recent developments has been a special tungsten filament in a gas-filled bulb placed at the focus of a small parabolic mirror. The beam is directed by means of sights and the flashes are obtained by interrupting the current by means of a trigger-switch. The filament is so sensitive that signals may be sent faster than the physiological process of vision will record. With the advent of wireless telegraphy light-signaling for long distances was temporarily eclipsed, but during the recent war it was revived and much development work was prosecuted.

The Ardois system consists of four lamps mounted in a vertical line as high as possible. Each lamp is double, containing a red and a white light, and these lights are controlled from a keyboard. A red light indicates a dot in the Morse code and a white light indicates a dash. The keys are numbered and lettered, so that the system may be operated by any one. Various other systems employing colored lights have been used, but they are necessarily short-range signals. Another example is the semaphore. When used at night, tungsten lamps in reflectors indicate the positions of the arms. The advantage of these signals over the flashing-system is that each signal is complete and easy to follow. The flashing-system is progressive and must be carefully followed in order to obtain the meaning of the dots and dashes.