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Or Photo Copying, the name given to that branch of photographic contact printing which is carried out without the aid of a camera-made negative. It is now used very extensively for copying documents, especially the plans of architects and engineers.

The earliest discovered process, the ferroprussiate, is still the one most largely used, on account of its economy and permanence, combined with a simplicity of manipulation that renders it highly suitable for office use; it was invented in 1840 by Sir John Herschel. This method has the disadvantage that the copies are blue in colour, and, as it is a negative process, the black lines of the original become the white lines of the print the development is by washing in water, so that the important feature of accuracy of scale is lost. The next step of importance was in 1864, when William Willis of Birmingham, the father of the inventor of the platinotype system of photographic printing, invented the aniline process. In this method a paper sensitized with bichromate of potassium is exposed to light,. with the document (generally a tracing) in front of it; the unprotected lines are bleached out, but the protected ones remain and are developed by contact with vapour of aniline, a subsequent washing for the removal of chemicals completing the print. For twenty years this process was successfully used with little opposition other than that of the blue prints previously referred to, and of the Pellet process, which gave a blue line on a white ground, the inventor being associated throughout with the firm of Vincent Brooks, Day & Son; but since that time a large number of other methods have come into use, some requiring a paper negative in the first instance and some not, but all much aided by improved methods of applying electric light. The earliest of these improved systems utilizing electric light was that invented by Mr B. J. Hall, whose photo-copier consists of two semi-circular glasses forming a cylinder, which may be revolved, and through which an arc lamp travels, while the tracing and sensitized paper are strapped to its outer surface.

Between 1900 and 1908 attention was chiefly directed to overcoming the variation of scale that is inevitable in all systems that require a final washing in water either for development or for the removal of chemicals; and at least four excellent systems have arisen. While Mr F. R. Vandyke was perfecting the system which he patented in 1901 and which has been adopted by the Ordnance Survey Department at Southampton, Messrs Vincen Day & Son were working along somewhat similar lines, the outcome of which was their "True-to-Scale Photo Litho" system. In both these methods a reversed positive print is secured on zinc, from which copies can be made in printer's ink of any colour by the usual lithographic method on almost any material that may be desired. The plates prepared by these. methods are so sensitive to light that excellent results can be secured from drawings made even on semi-transparent material such as drawing paper, and of course the plates when made are capable of alteration or addition and can be stored for reprints.

An admirable process had since been invented by MM. Dorel Freres of Paris, which is even more expeditious, and being less in prime cost is more suitable when only a small number of prints is required. In this case a large sheet of thin zinc is coated with chemically-treated gelatin, with the result that when a ferroprussiate print is pressed down on it either with the hand or by a roller the protected lines affect the gelatin in such a way that the parts that have been in contact with them receive a greasy ink while the remainder of the surface rejects it, so that a small number (not generally exceeding six) of very excellent prints can be secured. The inventors refrained from taking out a patent either in France or elsewhere, preferring to work their invention as a secret process, but the formula appears either to have leaked out or to have been discovered, so that the process is, .perhaps with slight variations, used under numerous names. With the aid of the various systems of rotary copiers, by which blue prints of almost any length can be secured, Dorel prints identical in scale with the originals have been made of the length of 22 feet. An interesting kindred process but with well defined variations is known as velography.

For the technical and chemical details of the various methods reference may be made to Ferric and Heliographic Processes by G. E. Brown (Dawbarn & Ward). (F. V. B.)