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copying, copy and camera

COPPER SULPHIDES.—There are two distinct sulphides of copper which may be pre pared. Copper and sulphide, when melted together, combine with vivid incandescence, and produce the subsulphide (cuprous sulphide), Cu, S. Cupric sulphide is found native as covellite, but can be artificially prepared by precipitating a salt of copper with sulphuretted hydrogen.

COPYING.—This term is usually applied to the copying of engravings, drawings, plans sketches, etc., with the camera. A suitable camera for this purpose is depicted in Fig. 140. The lens which should be employed in copying should be a landscape doublet or triplet, or a portrait combination. Single lenses can not be used without distorting the lines of the copy. It is advisable to use one possessing the least possible distortion. Abney rec ommends a Dallmeyer's D lens with a small stop. In photograph ing it is necessary that the copy and the screen of the camera be both perfectly upright, and in con sequence parallel to each other.

A good form of copying stand is shown in Fig. The light should fall direct upon the copy in a horizontal direction. If side lights be used the texture of the paper is clearly represented, giving the picture an unnatural, rough appearance. The light should also be a brilliant one; for Indian ink or sepia sketches

direct sunlight should be used. If it should be required to enlarge or reduce the image, informa tion will be found under Enlarging.

When copying it is very advisable that there should be as little vibration or movement as possible. To avoid this the camera and board to hold the copy is suspended as shown in Fig. 141. By this arrangement, if any part be acci dentally touched during exposure, it is of no con sequence as the whole moves together.

For copying pictures in black and white ordinary dry plates will answer the purpose, but are inferior to collodion wet plates, which are almost universally used for this purpose. They give better brilliancy and contrast, and the in creased exposure is in this case of no conse quence. A plain iodized collodion should be used, and the plates developed with the ferrous sulphate or the pyrogallic developer described in the collodion wet process.