Tests show that, in addition to its higher luminous efficiency, an arc of this character directs a greater percentage of the light into the effective angle of the mirror. The small source results in a beam of small divergence; in other words, the beam differs from a cylinder by only one or two degrees. If the beam consisted entirely of parallel rays and if there were no loss of light in the atmosphere by scattering or by absorption, the beam intensity would be the same throughout its entire length. However, both divergence and atmospheric losses tend to reduce the intensity of the beam as the distance from the search-light increases.

Inasmuch as the intensity of the beam depends upon the actual brightness of the light-source, the brightness of a few modern light-sources are of interest. These are expressed in candles per square inch of projected area; that is, if a small hole in a sheet of metal is placed next to the light-source and the intensity of the light pa.s.sing through this hole is measured, the brightness of the hole is easily determined in candles per square inch.

BRIGHTNESS OF LIGHT-SOURCES IN CANDLES PER SQUARE INCH

Kerosene flame 5 to 10 Acetylene 30 to 60 Gas-mantle 30 to 500 Tungsten filament (vacuum) lamp 750 to 1,200 Tungsten filament (gas-filled) lamp 3,500 to 18,000 Magnet.i.te arc 4,000 to 6,000 Carbon arc for search-lights 80,000 to 90,000 Flame arc for search-lights 250,000 to 350,000 Sun (computed mean) about 1,000,000

As the reflector of a search-light is an exceedingly important factor in obtaining high beam-intensities, considerable attention has been given to it since the practicable electric arc appeared. The parabolic mirror has the property of rendering parallel, or nearly so, the rays from a light-source placed at its focus. If the mirror subtends a large angle at the light-source, a greater amount of light is intercepted and rendered parallel than in the case of smaller subtended angles; hence, mirrors are large and of as short focus as practicable. Search-light projectors direct from 30 to 60 per cent. of the available light into the beam, but with lens systems the effective angle is so small that a much smaller percentage is delivered in the beam. Mangin in 1874 made a reflector of gla.s.s in which both outer and inner surfaces were spherical but of different radii of curvature, so that the reflector was thicker in the middle. This device was "silvered" on the outside and the refraction in the gla.s.s, as the light pa.s.sed through it to the mirror and back again, corrected the spherical aberration of the mirrored surface. These have been extensively used. Many combinations of curved surfaces have been developed for special projection purposes, but the parabolic mirror is still in favor for powerful search-lights. The tip of the positive carbon is placed at its focus and the effective angle in which light is intercepted by the mirror is generally about 125 degrees.

Within this angle is included a large portion of the light emitted by the light-source in the case of direct-current arcs. If this angle is increased for a mirror of a given diameter by decreasing its focal length, the divergence of the beam is increased and the beam-intensity is diminished. This is due to the fact that the light-source now becomes apparently larger; that is, being of a given size it now subtends a larger angle at the reflector and departs more from the theoretical point.

When the recent war began the search-lights available were intended generally for fixed installations. These were "barrel" lights with reflectors several feet in diameter, the whole output sometimes weighing as much as several tons. Shortly after the entrance of this country into the war, a mobile "barrel" search-light five feet in diameter was produced, which, complete with carriage, weighed only 1800 pounds. Later there were further improvements. An example of the impetus which the stress of war gives to technical accomplishments is found in the development of a particular mobile searchlight. Two months after the War Department submitted the problems of design to certain large industrial establishments a new 60-inch search-light was placed in production. It weighed one fifth as much as the previous standard; it had one twentieth the bulk; it was much simpler; it could be built in one fourth the time; and it cost half as much. Remote control of the apparatus has been highly developed in order that the operator may be at a distance from the scattered light near the unit. If he is near the search-light, this veil of diffused light very seriously interferes with his vision.

Mobile power-units were necessary and the types developed used the automobile engine as the prime mover. In one the generator is located in front of the engine and supported beyond the automobile cha.s.sis. In another type the generator is located between the automobile transmission and the differential. A standard clutch and gear-shift lever is employed to connect the engine either with the generator or with the propeller shaft of the truck. The first type included a 115-volt, 15-kilowatt generator, a 36-inch wheel barrel search-light, and 500 feet of wire cable. The second type included a 105-volt, 20-kilowatt generator, a 60-inch open searchlight, and 600 feet of cable. This type has been extended in magnitude to include a 50-kilowatt generator. When these units are moved, the search-light and its carriage are loaded upon the rear of the mobile generating equipment. An idea of the intensities obtainable with the largest apparatus is gained from illumination produced at a given distance. For example, the 15-kilowatt search-light with highly concentrated beam, produced an illumination at 930 feet of 280 foot-candles. At this point this is the equivalent of the illumination produced by a source having a luminous intensity of nearly 250,000,000 candles.

Of course, the range at which search-lights are effective is the factor of most importance, but this depends upon a number of conditions such as the illumination produced by the beam at various distances, the atmospheric conditions, the position of the observer, the size, pattern, color, and reflection-factor of the object, and the color, pattern, and reflection-factor of the background. These are too involved to be discussed here, but it may be stated that under ordinary conditions these powerful lights are effective at distances of several miles.

According to recent work, it appears that the range of a search-light in revealing a given object under fixed conditions varies about as the fourth root of its intensity.

Although the metallic parabolic reflector is used in the most powerful search-lights, there have been many other developments adapted to warfare. Fresnel lenses have been used above the arc for search-lights whose beams are directed upward in search of aircraft, thus replacing the mirror below the arc, which, owing to its position, is always in danger of deterioration by the hot carbon particles dropping upon it.

For short ranges incandescent filament lamps have been used with success. Oxyacetylene equipment has found application, owing to its portability. The oxyacetylene flame is concentrated upon a small pellet of ceria, which provides a brilliant source of small dimensions. A tank containing about 1000 liters of dissolved acetylene and another containing about 1100 liters of oxygen supply the fuel. A beam having an intensity of about 1,500,000 candles is obtained with a consumption of 40 liters of each of the gases per hour. At this rate the search-light may be operated twenty hours without replenishing.

Although the beacon-light for nocturnal airmen is a development which will a.s.sume much importance in peaceful activities, it was developed chiefly to meet the requirements of warfare. These do not differ materially from those which guide the mariner, except that the traveler in the aerial ocean is far above the plane on which the beacon rests.

For this reason the lenses are designed to send light generally upward.

In foreign countries several types of beacons for aerial navigation have been in use. In one the light from the source is freely emitted in all upward directions, but the light normally emitted into the lower hemisphere is turned upward by means of prisms. In a more elaborate type, belts of lenses are arranged so as to send light in all directions above the horizontal plane. A flashing apparatus is used to designate the locality by the number or character of the flashes. Electric filaments and acetylene flames have been used as the light-sources for this purpose. In another type the light is concentrated in one azimuth and the whole beacon is revolved. Portable beacons employing gas were used during the war on some of the flying-fields near the battle front.

All kinds of lighting and lighting-devices were used depending upon the needs and material available. Even self-luminous paint was used for various purposes at the front, as well as for illuminating watch-dials and the scales of instruments. Wooden b.u.t.tons two or three inches in diameter covered with self-luminous paint could be fixed wherever desired and thus serve as landmarks. They are visible only at short distances and the feebleness of their light made them particularly valuable for various purposes at the battle front. They could be used in the hand for giving optical signals at a short distance where silence was essential. Self-luminous arrows and signs directed troops and trucks at night and even stretcher-bearers have borne self-luminous marks on their backs in order to identify them to their friends.

Somewhat a.n.a.logous to this application of luminous paint is the use of blue light at night on battle-ships and other vessels in action or near the enemy. Several years ago a Brazilian battle-ship built in this country was equipped with a dual lighting-system. The extra one used deep-blue light, which is very effective for eyes adapted to darkness or to very low intensities of illumination and is a short-range light.

Owing to the low luminous intensity of the blue lights they do not carry far; and furthermore, it is well established that blue light does not penetrate as far through ordinary atmosphere as lights of other colors of the same intensity.

The war has been responsible for great strides in certain directions in the development and use of artificial light and the era of peace will inherit these developments and will adapt them to more constructive purposes.

XV

SIGNALING

From earliest times the beacon-fire has sent forth messages from hilltops or across inaccessible places. In this country, when the Indian was monarch of the vast areas of forest and prairie, he spread news broadcast to roving tribesmen by means of the signal-fire, and he flashed his code by covering and uncovering it. Castaways, whether in fiction or in reality, instinctively turn to the beacon-fire as a mode of attracting a pa.s.sing ship. On every hand throughout the ages this simple means of communication has been employed; therefore, it is not surprising that mankind has applied his ingenuity to the perfection of signaling by means of light, which has its own peculiar fields and advantages. Of course, wireless telephony and telegraphy will replace light-signaling to some extent, but there are many fields in which the last-named is still supreme. In fact, during the recent war much use was made of light in this manner and devices were developed despite the many other available means of signaling. One of the chief advantages of light as a signal is that it is so easily controlled and directed in a straight line. Wireless waves, for example, are radiated broadcast to be intercepted by the enemy.

The beginning of light-signaling is hidden in the obscurity of the past.

Of course, the most primitive light-signals were wood fires, but it is likely that man early utilized the mirror to reflect the sun's image and thus laid the foundation of the modern heliograph. The Book of Job, which is probably one of the oldest writings available, mentions molten mirrors. The Egyptians in the time of Moses used mirrors of polished bra.s.s. Euclid in the third century before the Christian era is said to have written a treatise in which he discussed the reflection of light by concave mirrors. John Peckham, Archbishop of Canterbury in the thirteenth century, described mirrors of polished steel and of gla.s.s backed with lead. Mirrors of gla.s.s coated with an alloy of tin and mercury were made by the Venetians in the sixteenth century. Huygens in the seventeenth century studied the laws of refraction and reflection and devised optical apparatus for various purposes. However, it was not until the eighteenth century that any noteworthy attempts were made to control artificial light for practical purposes. Dollond in 1757 was the first to make achromatic lenses by using combinations of different gla.s.ses. Lavoisier in 1774 made a lens about four feet in diameter by constructing a cell of two concave gla.s.ses and filling it with water and other liquids. It is said that he ignited wood and melted metals by concentrating the sun's image upon them by means of this lens. About that time Buffon made a built-up parabolic mirror by means of several hundred small plane mirrors set at the proper angles. With this he set fire to wood at a distance of more than two hundred feet by concentrating the sun's rays. He is said also to have made a lens from a solid piece of gla.s.s by grinding it in concentric steps similar to the designs worked out by Fresnel seventy years later. These are examples of the early work which laid the foundation for the highly perfected control of light of the present time.

While engaged in the survey of Ireland, Thomas Drummond in 1826 devised apparatus for signaling many miles, thus facilitating triangulation.

Distances as great as eighty miles were encountered and it appeared desirable to have some method for seeing a point at these great distances. Gauss in 1822 used the reflection of the sun's image from a plane mirror and Drummond also tried this means. The latter was successful in signaling 45 miles to a station which because of haze could not be seen, or even the hill upon which it rested. Having demonstrated the feasibility of the plan, he set about making a device which would include a powerful artificial light in order to be independent of the sun. In earlier geodetic surveys Argand lamps had been employed with parabolic reflectors and with convex lenses, but apparently these did not have a sufficient range. Fresnel and Arago constructed a lens consisting of a series of concentric rings which were cemented together, and on placing this before an Argand lamp possessing four concentric wicks, they obtained a light which was observed at forty-eight miles.

Despite these successes, Drummond believed the parabolic mirror and a more powerful light-source afforded the best combination for a signal-light. In searching for a brilliant light-source he experimented with phosphorus burning in oxygen and with various brilliant pyrotechnical preparations. However, flames were unsteady and generally unsuitable. He then turned in the direction which led to his development of the lime-light. In his first apparatus he used a small sphere of lime in an alcohol flame and directed a jet of oxygen through the flame upon the lime. He thereby obtained, according to his own description in 1826,

a light so intense that when placed in the focus of a reflector the eye could with difficulty support its splendor, even at a distance of forty feet, the contour being lost in the brilliancy of the radiation.

He then continued to experiment with various oxides, including zirconia, magnesia, and lime from chalk and marble. This was the advent of the lime-light, which should bear Drummond's name because it was one of the greatest steps in the evolution of artificial light.

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 a.n.a.logous 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 semaph.o.r.e. 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.

Smaller signal-lamps using acetylene have been employed in the forestry service and in other activities where a portable device is necessary. In one type, a mixture-tank containing calcium carbide and water is of sufficient capacity for three hours of signaling. A small pilot-light is permitted to burn constantly and the flashes are obtained by operating a key which increases the gas-pressure. The light flares as long as the key is depressed. The range of this apparatus is from ten to twenty miles. An electric lamp supplied from a storage battery has been designed for geodetic operations in mountainous districts where it is desired to send signals as far as one hundred miles. Tests show that this device is a hundred and fifty times more powerful than the ordinary acetylene signal-lamp, and it is thought that with this new electric lamp haze and smoke will seldom prevent observations.

Certain fixed lights are required by law on a vessel at night. When it is under way there must be a white light at the masthead, a starboard green light, a port red light, a white range-light, and a white light at the stern. The masthead light is designed to emit light through a horizontal arc of twenty points of the compa.s.s, ten on each side of dead ahead. This light must be visible at a distance of five miles. The port and starboard lights operate through a horizontal arc of twenty points of the compa.s.s, the middle of which is dead ahead. They are screened so as not to be visible across the bow and they must be intense enough to be visible two miles ahead. The masthead light is carried on the foremast and the range-light on the mainmast, at an elevation fifteen feet higher than the former. The range-light emits light toward all points of the compa.s.s and must be intense enough to be seen at a distance of three miles. The stern light is similar to the masthead, but its light must not be visible forward of the beam. When a vessel is towing another it must display two or three lights in a vertical line with the masthead light and similar to it. The lights are s.p.a.ced about six feet apart, and two extra ones indicate a short tow and three a long one. A vessel over a hundred and fifty feet long when at anchor is required to display a white light forward and aft, each visible around the entire horizon. These and many other specifications indicate how artificial light informs the mariner and makes for order in shipping.

Without artificial light the waterways would be trackless and chaos would reign.

The distress signals of a vessel are rockets, but any burning flame also serves if rockets are unavailable. Fireworks were known many centuries ago and doubtless the possibilities of signaling by means of rockets have long been recognized. An early instance of scientific interest in rockets and their usefulness is that of Benjamin Robins in 1749. While he was witnessing a display of fireworks in London it occurred to him that it would be of interest to measure the height to which the rockets ascended and to determine the ranges at which they were visible. His measurements indicated that the rockets ascended usually to a height of 440 yards, but some of them attained alt.i.tudes as high as 615 yards. He then had some special ones made and despatched letters to friends in three different localities, at distances as great as 50 miles, asking them to observe at a certain time, when the rockets were to be sent up in the outskirts of London. Some of these rockets rose to alt.i.tudes as great as 600 yards and were distinctly seen by observers 38 miles away.

Later he made rockets which ascended as high as 1200 yards and concluded that this was a practical means of signaling. Since that time and especially during the recent war, rockets have served well in signaling messages.

The self-propelled rockets have not been altered in essential features since the remote centuries when the Chinese first used them in celebrations. A cylindrical sh.e.l.l is mounted on a wooden stick and when the powder in the sh.e.l.l burns the hot gases are ejected so violently downward that the reaction drives the sh.e.l.l upward. At a certain point in the air, various signals burst forth, which vary in character and color. One of the advantages of the rocket is that it contains within itself the force of propulsion; that is, no gun is necessary to project it. The illuminating compounds and various details are similar to those of the illuminating sh.e.l.ls described in another chapter.

At present the rocket is not scientifically designed to obtain the greatest efficiency of propulsion, but its simplicity in this respect is one of its chief advantages. If the self-propelled rocket becomes the projectile of the future, as some have ventured to predict, much consideration must be given to the design of the orifice through which the gases violently escape in order that the best efficiency of propulsion may be attained. There are other details in which improvements may be made. The combustion products of the black powder which are not gaseous equal about one third the weight of the powder.

This represents inefficient propulsion. Furthermore, during recent years much information has been gained pertaining to the air-resistance which can be applied to advantage in designing the form of rockets.

Besides the various rockets, signal-lights have been constructed to be fired from guns and pistols. During the recent war the airman in the dark heights used the pistol signal-light effectively for communication.

These devices emitted stars either singly or in succession, and the color of these stars as well as their number and sequence gave significance to the signal. Some of these light-signals were provided with parachutes and were long-burning; that is, light was emitted for a minute or two. There are many variations possible and a great many different kinds of light-signals of this character were used. In the front-line trenches and in advances they were used when telephone service was unavailable. The airman directed artillery fire by means of his pistol-light. Rockets brought aid to the foundered ship or to the life-boats. The signal-tube which burned red, green, or white was held in the hand or laid on the ground and it often told its story. For many years such a device dropped from the rear of the railroad train has kept the following train at a safe distance. A device was tried out in the trenches, during the war, which emitted a flame. This could be varied in color to serve as a signal and the apparatus had sufficient capacity for thirty hours' burning. This could also be used as a weapon, or when reduced in intensity it served as a flash-light.

For many years experiments have been made upon the use of the invisible rays which accompany visible rays. The practicability of signaling with invisible rays depends upon producing them efficiently in sufficient quant.i.ty and upon separating them from the visible rays which accompany them. Some successful results were obtained with a 6-volt electric lamp possessing a coiled filament at the focus of a lens three inches in diameter and twelve inches in focal length. This gave a very narrow beam visible only in the neighborhood of the observation post to which the signals were directed. The beam was directed by telescopic sights.

During the day a deep red filter was placed over the lamp and the light was invisible to an observer unless he was equipped with a similar red screen to eliminate the daylight. It is said that signals were distinguished at a distance of six miles. By night a screen was used which transmitted only the ultraviolet rays, and the observer's telescope was provided with a fluorescent screen in its focal plane. The ultraviolet rays falling upon this screen were transformed into visible rays by the phenomenon of fluorescence. The range of this device was about six miles. For naval convoys lamps are required to radiate toward all points of the compa.s.s. For this purpose a quartz mercury-arc which is rich in ultraviolet rays was surrounded with a chimney which transmitted the ultraviolet rays efficiently and absorbed all visible rays excepting violet light. The lamp appeared a deep violet color at close range, but the faintly visible light which it transmitted was not seen at a distance. A distant observer picks up the invisible ultraviolet "light" by means of a special optical device having a fluorescent screen of barium-platino-cyanide. This device had a range of about four miles.

Light-signals are essential for the operation of railways at night and they have been in use for many years. In this field the significance of light-signals is based almost universally on color. The setting of a switch is indicated by the color of the light that it shows. With the introduction of the semaph.o.r.e system, in which during the day the position of the arm is significant, colored gla.s.ses were placed on the opposite end of the arm in such a manner that a certain colored gla.s.s would appear before the light-source for a certain position of the arm.

A kerosene flame behind a gla.s.s lens was the lamp used, and, for example, red meant "Stop," green counseled "Caution," and clear or white indicated "All clear." For many years the kerosene lamp has been used, but recently the electric filament lamp is being installed to some extent for this purpose. In fact, on one railroad at least, tungsten lamps are used for light-signals by day as well as by night. Three signals--red, green, and white--are placed in a vertical line and behind each lens are two lamps, one operating at high efficiency and one at low efficiency to insure against the failure of the signal. The normal daylight range is about three thousand feet and under the worst conditions when opposed to direct sunlight, the range is not less than two thousand feet. It is said that these lights are seen more easily than semaph.o.r.e arms under all circ.u.mstances and that they show two or three times as far as the latter during a snow-storm.

The standard colors for light-signals as adopted by the Railway Signal a.s.sociation are red, yellow, green, blue, purple, and lunar white. These are specified as to the amount of the various spectral colors which they transmit when the light-source is the kerosene flame. Obviously, the colors generally appear different when another illuminant is used. The blue and purple are short-range signals, but the effective range of the best railway signal employing a kerosene flame is only about four miles.

It has been shown that the visibility of point sources of white light in clear atmosphere, for distances up to a mile at least, is proportional to their candle-power and inversely proportional to the square of the distance. Apparently the luminous intensities of signal-lamps required in clear weather in order that they may be visible must be 0.43 candles for one nautical mile, 1.75 candles for two nautical miles, and 11 candles for five nautical miles. From the data available it appears that a red or a white signal-light will be easily visible at a distance in nautical miles equal to the square root of its candle-power in that direction. The range in nautical miles of a green light apparently is proportional to the cube root of the candle-power. Whether or not these relations between the range in miles and the luminous intensity in candles hold for greater distances than those ordinarily encountered has not been determined, but it is interesting to note that the square root of the luminous intensity of the Navesink Light at the entrance to New York Harbor is about 7000. Could this light be seen at a distance of seven thousand miles through ordinary atmosphere?

The most distinctive colored lights are red, yellow, green, and blue. To these white (clear) and purple have been added for signaling-purposes.

Yellow is intense, but it may be confused with "white" or clear. Blue and purple as obtained from the present practicable light-sources are of low intensity. This leaves red, green, and clear as the most generally satisfactory signal-lights.

There are numerous other applications, especially indoors. Some of these have been devised for special needs, but there are many others which are general, such as for elevators, telephones, various call systems, and traffic signals. Light has the advantages of being silent and controllable as to position and direction, and of being a visible signal at night. Thus, in another field artificial light has responded to the demands of civilization.

XVI