Photography is the art of making pictures by the direct action of light on a sensitive surface dates from the beginning of the 19th century. The action of light on certain salts of silver was studied by Scheele, a Swedish chemist, in 1777, who laid the foundation for the work which followed. He found that certain of the salts were blackened by exposure to light, the effect being due largely to the blue and violet rays in the spectrum.
It was an easy step to coat paper with this sensitive substance and obtain impressions of leaves, ferns and similar objects by the action of sunlight; but no way was known of rendering the pictures permanent and they attracted no great amount of attention. In 1839 a Frenchman named Daguerre succeeded in producing a sensitive surface on a copper plate which was so rapidly affected by light that impressions could be made on it in the camera, which impressions could be made permanent.
The process at once met with great favor, and, owing to its comparative cheapness, almost immediately supplanted miniature painting, which was then in vogue. The disadvantages of Daguerre's process were manifold. Very long exposures were required, a sitting was required for each picture, and the picture had to be held in a certain light to be seen.
The next step was the invention of the collodion plate, which shortened the time of exposure, and furnished the means of producing any number of pictures from a single plate. It was necessary to use these plates in a wet condition, however, and their employment was consequently limited to the studio. The discovery of the dry plate, which is in use at the present time, was made about a quarter of a century ago and was the means of bringing the art within the reach of everybody. Before the days of dry plates the photographer was obliged to carry about with him a portable dark room, for the plates had to be immersed in a bath of silver nitrate immediately before exposure, and the development could not be postponed a moment after exposure.
The present dry-plate process is essentially as follows: Glass plates are coated with a film of gelatine containing a mixture of bromide and iodide of silver. After exposure in the camera they are developed by means of suitable chemicals in a room illuminated only with red light. The action of the light on the plate is to start a reaction in the silver salt which requires the action of the developer to complete it. This reaction is the transformation of the white bromide of silver into black metallic silver.
The plate after development consequently appears black wherever the light has acted, on it, the resulting picture being called a negative, since the high lights are black and the deep shadows white. After development the plate is transferred to a bath of hyposulphite of soda, which dissolves the unaffected silver-salts leaving the gelatine quite transparent except foi the black deposit which forms the picture. From this negative any number of pictures can be printed by exposing sheets of sensitized paper under it to the action of sunlight.
The black deposit of silver in the film screens the paper from the blackening action of the light; consequently the resulting print is white in those places which, in the negative, are dark, and the picture is a positive. Modern plates are made so sensitive that it is possible to secure pictures of objects in sunlight in the 1-1,000 part of a second. The plates of Daguerre required an exposure of from five to 15 minutes.
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Since the introduction of these extremely sensitive plates photography has proved of the greatest aid in scientific investigations. In astronomy clusters of stars and nebulae have been photographed, which no eye can see, even in the most powerful telescopes, for the photographic plate can be exposed for hours to the image, the action of the feeble light accumulating in the sensitive film, whereas in the eye, if the light is too feeble at once to affect the retina, prolonged gazing is wholly without effect.
By employing the light of the electric spark, rapidly moving objects can be photographed, in as brief an interval as the millionth part of a second. In this way beautiful pictures of flying rifle-balls, witn the ripples and waves of air which accompany them and the boiling wake which follows them, have been secured by Professor Boys of London. Professor Wood of the University of Wisconsin had in a similar manner secured pictures of sound-waves 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 future 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.
COLOR PHOTOGRAPHY. 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 by this method has thus far been confined to the laboratory, the conditions for success not yet being quite fully understood. Probably not more than half a dozen people in the world have succeeded in getting satisfactory results, owing to the experimental difficulties.
Of the indirect processes there are a number which have been developed far enough to be considered commercial successes. These processes are all based on the principle that any color can be imitated by a mixture of the three primary colors, red, green and blue. The most beautiful results have been obtained by Mr. Ives of Philadelphia, one of the pioneers in ^ this line of work, whose process consists, briefly, in taking three negatives of the subject through red, green and blue glasses. These glasses prevent light of any other color than that which they are designed to transmit from getting at the plate; consequently each negative is a record of the distribution of one of the three primary colors in the original. From these negatives three transparencies on glass are printed, which, when thrown superimposed on a screen by means of three lanterns furnished with colored glasses similar to the ones used in taking the pictures, combine to form a very perfect reproduction of the original.
For exhibiting the pictures without the lanterns Mr, Ives has devised an instrument which he has named the kromskop, in which the colored images are combined by reflectors. The pictures are stereoscopic as well, and the result is a reproduction so perfect that .it is almost impossible to believe that we are not looking at the object itself, for it stands out in full perspective with every color perfect.
Another process, which was perfected independently and almost simultaneously by Dr. Joly of Edinburgh and Mr. Mc-Donough of Chicago has been recently put upon a commercial basis. In this process the negative is taken on a plate in front of which is placed a screen ruled with very fine lines in red, green and blue ink, the colors following each other in succession across the screen. This screen breaks up the picture into linear strips, any one of which may be regarded as a record of one of the three primary colors along that portion of the picture. From the negative obtained in this way a positive is printed on glass, which, when mounted in contact with a similar tricolor ruled screen, reproduces the colors of the original. The objection to this method is the obtrusiveness of the lines, especially when the pictures are projected. The colors are much weaker and less faithfully reproduced than by the kromskop method.
Another method, devised originally by Prof. N. W. Wood and recently improved and perfected by Mr. Ives and his son, employs the diffraction grating as the source of color in the picture. The pictures can be duplicated by a purely mechanical process, but are colorless except, when examined with a special viewing apparatus. Results have been obtained by this method very nearly if not quite equal to those yielded by the kromskop. The most recent process, and the only one which appears to have been a commercial success, is the recently devised starch-grain process invented by the Lumiere brothers of Lyons. It is a modification of the Joly process, the colored lines being replaced by stained granules of potato-starch. The plates are not very difficult to operate, and the results are highly satisfactory, though by no means equalling those yielded by the kromskop method. R. W. WOOD.
Photo-Engraving is a process for the conversion of a photograph into an engraving, from which engraving prints may be taken by any good printing-press. Such perfection has been reached that photoengravings have largely replaced wood-engravings for illustrating books and newspapers. One of the best, if not most of the processes in use to-day, will be found based upon discoveries in the early part of the 19th century that asphaltum, when it has been subjected to the action of light, is no longer soluble in its ordinary solvents. A plate therefore coated with asphaltum which has been exposed in the camera obscura to light and shade would possess a surface, part of which was soluble and part insoluble. By the application of biting acids, these plates are then chemically etched. This process is especially useful to reproduce line-engravings or the pen-and-ink sketches used so freely in the daily papers which are now rapidly produced by chemical processes. Many of the variations in the ordinary process are kept secret, and others could hardly be explained to any other than an expert.
Photogravure. The expense of photogravure work greatly limits its use mostly to high-class bookwork. Large pictures, however, are produced by it which rival the finest steel engraving in finish and delicacy. The photographs can be reproduced, but the process is largely employed for obtaining engravings, such as copies of celebrated pictures. The process is so nearly perfect that any touch of the painter's brush is clearly seen in the copy. We here give one of the methods. (See PHOTOGRAPHY for sensitized plates, negative etc.) A gelatine relief is obtained by exposing bichromated gelatine to the action of 1'ght beneath a negative. The gelatine is mixed with a quantity of black lead in more or less granular form. This causes the relief to have a surface which is granular in character, and also makes it a conducting one for electricity. Put now into an electrotype bath; it will soon be covered with a deposit of copper. A copper printing-plate is thus made, from which pictures are printed.
The New Student's Reference Work (1914) pp. 1476-1478.