COPYING, a term applied in photography to the reproduction of paintings, engrav ings, manuscripts, maps, etc. The kinds of camera and lens most suitable for the pur pose, will be found described under their respective heads; the quality and condition of chemicals necessary are based upon the facts, that long ceposure is almost invariably required, and that, in the majority of cases, it is desired to copy black marks upon a white ground, as in a sheet of music, for example. Where it is obvious that nothing that can be called a middle tint is required, but simply pure black and white, recourse should be had to organic matter in the bath; a little acetate of soda, and an extra amount of acetic acid, may be also added, and an old collodion containing free iodine employed.
It is important that the work or surface to be copied should be placed in a strong light, and exactly at right angles to the axis of the lens, which should be furnished with a small stop. These three conditions, it will be seen, are such as are calculated to insure density in the blacks of the negative, freedom from distortion, and sharpness at the edges of the picture. The copying of oil-paintings seems to the amateur, at first sight, to present almost insuperable difficulties, on account of the reflected light from time varnish passing through the lens, and producing black patches on the negative. This may, however, be completely avoided by the employment of a lens of long focus, which admits of the oblique pencils of light passing off without entering the camera.
Attention to the laws of the reflection of light, will suggest to the reader the impor tance also of avoiding a bright light immediately behind the camera, as the rays of light would then fall on the varnished surface, nearly at right angles, and be reflected into the camera. The oil-painting, therefore, though placed in strong sunshine, for the pur pose of giving vigor to the inure obscure parts, should be so arranged as to allow the light to fall on it at an angle of about 35° or 40°.
In copying transparent negatives, a somewhat different arrangement is required, as will appear from considering the following facts. Every object to be copied may be regarded, for the sake of illustration, asap assemblage of bright points, from each of which divergent pencils of rays are reflected, and suffer refraction on passing through the lens:, an engraving or oil-painting is, in fact, in its relation to the sensitive surface, the source of light. In a negative, however, many of the parts of which are transparent glass, it is manifest the case is different, for if we suppose the sun or a luminous background to be placed behind the negative, that will act as the source of light, and any rays coming therefrom will pass almost directly through those parts of the negative which are bare glass, to the lens; thus producing the same effect as if the transparent parts were opaque, but luminous, and emitted divergent pencils of light. It is necessary, therefore, that the rays should he made to intersect at those points where bare glass exists, and this may be accomplished by employing what is called a condensing lens, by which means negatives may be most successfully copied, by placing an artificial light behind it, or still better, by reflected sunshine through it.
Negatives are sometimes copied on glass by direct superposition in the ordinary pressure-frame, such as is used for printing photographs on paper, in which case, dry plates are used, prepared either by the albumen or collodio-albumen process; and the latent image so obtained yields a transparent positive when developed by gallic or pyro gallic acid.