PHOTO-ENGRAVING                               1476                                       PHOTOGRAPHY

Pho'to=Engrav'ing 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 a.sphaltum 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.

Photog'raphy. 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.

Since the introduction of these extremely sensitive plates photography has proved of the greatest aid in scientific investigations. In astronomy clusters of stars and nebulas 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, with 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