COPYING ; RESTORATIONS TO THE VERTICAL ; DEFORMATIONS 733. Copying by Contact Printing. An original on paper of homogeneous structure, even though rather thick, may be copied by transmitted light, provided the back of the paper bears no imprint or note. This is often the quickest way of reproducing originals. It is useless, and often dangerous, to oil the paper (§ 481) in order to increase its translucency, for the only advan tage is a slight reduction of the time of exposure, and this is more than balanced by the time taken to oil the paper and then free it from the grease.
It must be borne in mind that the contrasts of an image are always considerably less when it is examined by transmitted instead of reflected light. This loss of contrast must be compensated for by choosing a slow emulsion capable of yielding a contrasty image by suitably prolonged development.
These same working methods are applicable to the copying of the structure of leaves or of anatomical sections. 2 734. Copying by Contact in Reflected Light. The possibility of copying by contact a black and white original (not in tones), either opaque or with an imprint on the back, was pointed out as long ago as 1839 by A. Breyer, and the method to be used was described by P. Yvon (1891). 3 A yellow or red filter is placed in a printing frame, and on it is laid the sensitive surface (plate, film, or paper), with the emulsion side towards the inside of the frame. The face of the original to be copied is laid on the emulsion. A sheet of black paper is placed over the original, and the printing frame is closed and exposed to light for a time which has been ascertained.' Under these conditions, the light coming to the emulsion before reaching the original tends to fog it uniformly, but a portion of the incident light (the greater as the light used is less active) passes through the sensitive coating and reaches the image to be copied. This light is absorbed by the black portions and diffused by the white ones, which latter return it to the sensitive emulsion. Very approximately it may be said that opposite the white areas the emulsion receives twice as much light as opposite the black ones. By using a very contrasty, non
orthochromatic and an energetic developer heavily dosed with bromide (§ 386), a negative is thus obtained which may not be perfect, but is quite usable, especially after superficial reduction and vigorous intensification.
This method was for a long time used only exceptionally for preparing lantern slides from illustrations in scientific or technical works. It has acquired a certain industrial importance, particularly for reprints of books, since M. Ullmann (1913) suggested the substitution of a thin layer of bichromated gelatine for the sensitive emulsion. By suitably regulating the exposure, the gelatine is rendered insoluble oppo site the whites and remains soluble opposite the blacks. After washing away the soluble portions, the relief of colourless gelatine can be brought to any required density by dyeing or by the form ation of opaque precipitates (Manul process).
735. Copying with a Camera. Although, in principle, copying may be undertaken with any camera having a sufficiently long extension, the adjustment of exact parallelism between the planes of the original and of the sensitive surface is extremely tedious, especially if the copy requires to be made to a given scale, unless specially built apparatus (§§ 150 to 152) is avail able to facilitate the work.
Some vertical enlargers (§ 762) can occasion ally be used as copying cameras. The easel and the dark slide are in an horizontal plane, and the axis of the lens is vertical. The parallelism of the two conjugate planes can then be tested very simply by means of a spirit level.
In the absence of a permanent installation, it is always possible to make such arrangements as obviate the necessity for the repeated adjust ments otherwise needed on each occasion. It is, for instance, possible to rig up a simplified copying bench by mounting the camera on a stool which can be slid along grooves or rails placed on a table, marks being drawn that permit of the table being always brought to the same position as regards the easel fixed permanently to a wall, and of the camera being always replaced in the same position on its stool.