Either daylight or artificial light may be employed. If the former is preferred, the apparatus should be placed on a, firm table in front of and at right angles to a window, if possible, facing north. This will give a strong side-light, and would, when copying engravings or photographs give a, very bad effect, the mode of lighting exaggerating the grain of the paper. To remedy this, a small bedroom mirror should be placed on the shadow-side of the picture, and so adjusted as to throw as much reflected light as possible on the picture to be copied. A picture on even rough paper should not show any marked granularity when lighted in this way. The same system is employed when copying by lamp or gaslight. The illuminants in this case, whether gaslights or oil-lamps, are placed on both sides of the apparatus, and in such a position that neither source of light is included in the field of the lens, and it is well, in any ease, to place a cardboard or other screen between tho source of light and the lens. The proper amount of exposure can only be learnt by experience, though after very little practice, the conditions being much more uniform than when photographing in the field, it will be a compara tively easy matter to determine it correctly.
The excellence of the result will depend to some extent upon the plate which is used. Now, an extra rapid plate is the worst possible for this class of work, and the slowest obtainable should be chosen. There are plates specially made for the purpose, called photo-mechanical plates. Some of these should be obtained, as good results will be more easily obtainable upon them than upon ordinary plates. They are very slow, probably not half so rapid as ordinary plates. In copying line engravings the great point to aim at is to secure sufficient density in the opaque portions of the negative, at the same time keeping the lines clear and free from fog. In other words, we want great opacity combined with extreme clearness. These qualities will not be easily obtainable with quick plates, but will not be diffi cult to secure on the specially-prepared plates to which I have referred. An exposure as nearly accurate as possible having been given, it becomes, of course, necessary to deve lop the plate. Now, undoubtedly, the best developer to
employ for this particular work is hydroquinone, and its tendency, when used for ordinary negative work, to give extreme density and contrast becomes, for this special purpose, an advantage rather than a Lfect. Equal por tions of the stock solutions, directions for preparing which were given in the chapter on the preparation of the solutions, should be poured into a clean developing cup, and half its bulk of water added. This should then be applied to the plate. If the exposure has been approximately correct, the image will appear slowly and regularly, and development must be continued until a dense and vigorous negative is obtained. If the plate has been properly ex posed, the clear portions of the negatives should remain unclouded and free from veil or fog until the last. If, however, they begin to veil before sufficient density is obtained, the exposure has been excessive, and the plate must be removed from the developer, washed, fiAed, and, if a second plate cannot be exposed, which is by far the best plan, the over-exposed one should be intensified ; but with the photo-mechanical plates it should not be necessary to resort to intensification if the exposure has been correct.
In copying silver prints it is better, when possible, to have them enamelled, as there will be less granularity in the copy. Care, however, must be taken to avoid reflected light from the glazed surface ; this can be easily managed by adjusting the mirror or reflector. Unmounted prints are best soaked in clean water, and carefully squeegeed to a sheet of glass; not, of course, face downwards, as to photo graph through the glass would probably give distortion, and reflections would be difficult to avoid. This mode of treat ment takes out all creases, and does not injure the photo graph. Bromide and platinotypes may, if desirable, be treated in the same way.
When coloured pictures, water-colours, oil paintings, or chromo-lithograplis have to be copied, the use of isochro matic plates becomes indispensable; but as their employment confers such great advantages, I propose to devote the next chapter to a description of their special properties and use.