In the ozotype process perfected by Manly, paper is coated with a patented sensitive solu tion consisting of potassium bichromate and certain other salts, dried and printed as in gum-bichromate; and may then be kept indefi nitely. To develop the print the printed paper, with a slightly visible image, is soaked in a solution of hydroquinone acetic acid, copper or iron sulphate, etc., according to the effect de sired, and, under the solution, brought into contact with a piece of carbon tissue, or the •plaster') prepared by the patentee. Develop ment takes place in warm water, the coloring matter of the plaster or tissue adhering to the parts of the print acted on by light. The ferro prussiate process (blue print process) was dis covered in 1842 by Sir John Herschel.
Not the least important of what may be called the side issues, or secondary applications of photography, are the various productions used in printing. See PHOTO-ENGRAVING.
Photography in natural colors, or, as Sir W. de W. Abney has it, in the colors of nature, has been the dream of many experimenters; but, notwithstanding all that has been doae, we are no nearer it than when they began. Color photography, however, that is, photographs hav ing the semblance of the natural colors, has made considerable progress. Becquerel was the first to secure on silver chloride something ap proaching the colors of the spectrum, but- got no further; and to Ducos du Haroun is due the credit for, in 1869, clearly foreshadowing the three methods which include all that has as yet been done in it — the superimposing of three-color images, Joly-McDonough colored lines, and Lippmann's interference process. Taking them in the order of their least im portance, Lippmann's method is to expose a very thin sensitive film backed by mercury as a reflector, to the colored object. Incident light reflected from the metallic mirror in con tact with the film results in interference, and, as the constituents of white light are of varied wave-length, produces in the film a series of planes parallel with its surface, emitting colored light exactly as does the soap-bubble; but the process is difficult, and not likely ever to be more than a scientific curiosity. In the Joly McDonough method a negative is made in the ordinary way, but with a glass plate with closely ruled colored lines in front of and in contact with the sensitive plate. From the negative so made a positive is printed, and a second or viewing screen with similar colored lines is placed in contact with it, and in exact register with the impressed lines, the result being a picture in the semblance of the natural colors.
An improvement over the screen color lined plate was made in 1906 by August and Louis Lumiere of Lyons, France, manufacturers of plates and films, by the introduction of single glass plate coated with a special transparent film, upon which is sprinkled a composite mixture of colored microscopic dust-like starch (potato starch) grains, colored respectively orange, green and violet, there about 5,000,000 colored grains to the square inch. After the
plate is thus coated it is brought under pressure by special mechanical means which flattens out the minute colored starch grains, causing them to merge into each other, giving the appear ance, under the microscope, of a mosaic forma tion. Viewed by transmitted light, the screen appears to have no color.
Upon the screen film thus formed the ortho chromatic silver sensitized gelatine emulsion is flowed, and when dry the plate is packed ready for use in the camera, like an ordinary dry plate, except that it is inserted in the plate holder film side down, against a sheet of black surfaced paper, which comes with a box of plates. Thus the glass side of the plate is next to the lens.
A special yellow colored filter intended to absorb a portion of the blue rays of light is interposed in the camera between the lens and the plate. The light from the object to be photographed, after passing through the lens and color filter, first impinges upon the glass side of the color sensitive plate, then pene trates the screen film and lastly acts upon the back of the sensitized film, affecting the film automatically in proportion as the color par ticles of the screen film transmit the colors of the object photographed to the sensitized film.
The exposure of the plate in the camera for any given stop or diaphragm is usually about 50 times longer than for an ordinary fast plate. The developer used is of the metol-quinone type, having liquid ammonia as an accelerator.
After exposure, the developer (at a tempera ture of between to 65° F.) is applied to the plate (placed in a tray) preferably in a room that is perfectly dark, for two and a half minutes. It is then poured off, the plate rinsed with water, then a reversing solution (perman ganate of potassium) is applied (under a bright light) for three minutes which dissolves away the black reduced silver negative image, con verting the same into a positive image. The plate is next rinsed under the tap and the same developer is again used in the tray a second time in bright daylight, which converts the un reduced bromide of silver (or what would represent the shadows in the original negative image) into dark metallic silver, and thus com pletes the manipulation required to make the transparency. The plate, after removal from the developer, is washed under the tap for a brief period, and, on viewing the same by transmitted daylight, a beautifully colored transparency, possessing all the gradations of the color of the original, is observed.