PHOTOGRAPHY
1477
PHOTOGRAPHY
in air, the image of the spherical wave of condensed air being impressed on the photographic plate by the light of an electric spark occurring at just the right moment. By means of cameras fitted with electric lights, which have been lowered into the sea, pictures of the ocean's bottom, with the sea-plants and coral formations, have been taken. Swung from the tails of kites, cameras, operated by an electric current sent up the wire kite-string, give us pictures of our surroundings as they appear from an elevation of a mile or two.
Book and magazine illustration is now done largely by photography, the old-fashioned woodcut having been driven out by the zinc plate, which is engraved or etched by a photographic process, giving an absolute fac-simile of the original drawing. One of the most remarkable developments within the last ten years has been the production of plates which are sensitive to all the colors of the spectrum. The ordinary commercial plates are sensitive only to the blue and violet parts of the spectrum, consequently red or yellow objects always come out black in the finished picture. Vogel of Berlin discovered, however, that if the plates were slightly stained with some aniline dye capable of absorbing the red and yellow light, they at once became sensitive to these colors; consequently such plates could be used for photographing colored objects, where it was ssential that correct color-values should be rendered. Plates are now made which will blacken in the light of the ordinary dark-room's red lamp almost as quickly as ordinary plates in candle light. Such plates are called arthochromatic plates, and they are used for photographing paintings and other colored objects as well as in many of the processes of color photography. Great improvements have been made also in photographic lenses within the past quarter of a century, the firm of Zeiss in Jena having been most active in the development of the modern photographic objective.
It is impossible to predict what the f- ture has in store for photography. Still more sensitive plates would be of immense use, particularly in scientific photography, and it is not by any means impossible that some new discovery may at any time give us a plate ten times as rapid as the present one. What is most desired, however, is some satisfactory color-process, which can hardly be expected until some one is fortunate to discover that unknown chemical which has the property of assuming a color similar in hue to the color of the light which illuminates it, and retaining that color permanently, a discovery of which there is no immediate promise.
CQLOR PHOTOGRAPH? At the very beginning of the art of photography it was observed that traces of
color sometimes appeared in the picture which bore some resemblance to the color of the light acting on the plate. Photographs of the solar spectrum were obtained in this way at the beginning of the last century, in which the colors were reproduced with more or less fidelity, but no method was discovered of rendering the colors permanent, and it is only within the last few years that satisfactory methods have been devised of producing colored pictures by the aid of photography. The methods in use at the present time may be divided into two classes: The direct, in which the color is produced by the action of the light, and the indirect, in which the color is applied subsequent to the taking of the picture, the photographic process being modified so as to cause the colors to distribute themselves properly in the finished picture.
The only successful process of the first class is that of Lippmann, the French physicist, whose method was carefully worked out by theory before a single experiment was tried. Lippmann's process essentially is as follows: A photographic plate is placed in a holder with the glass-side toward the lens of the camera, and mercury is poured into the back of the holder, forming a metallic mirror in contact with the sensitive surface of the plate. The light after passing through the film is reflected back through the film in the opposite direction by the quicksilver mirror. A very remarkable thing now takes place. The reflected light-waves from the mirror interfere with the oncoming waves, producing what are known as stationary waves in the sensitive film. Now, while ordinary lightwaves deposit the silver in the film in a solid mass, the stationary waves have the singular power of depositing it in exceedingly thin laminae of films, each one thinner than the thinnest gold leaf. The thickness of the silver laminae varies with the color of the light producing them, red light or long waves producing thicker films than blue light or short waves. Thin films, we know, show brilliant colors by reflected light, the commonest example being the soap-bubble, and the silver laminae in Lippmann's pictures show color in exactly the same way. The color of any part of the bubble depends on the thickness of the soap-film, and in the same way the color of any portion of the photograph depends on the thickness of the silver-films deposited by the light-waves in that place. The curious thing about the process is that light of any given color will deposit films of just the right thickness to showfehe same color by reflected light after the plate is developed and fixed in the usual manner. What is still more remarkable is that this fact was recognized by the inventor of the process before any of his experimental work was done. The production of pictures
