This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Artificial Light. While the best results are obtained by daylight yet it is possible to copy at night by lamp, electric, or gaslight. The arrangements, however, are practically the same, except that with artificial light two lamps should be employed, one at each side. This will give a more even illumination. Each light should be partly surrounded by a sheet of white cardboard bent in semi-circular shape, and held in that position with a piece of thread around the top and bottom. This reflects light onto the object and prevents any light shining into the lens. If kerosene lamps are employed they may be placed on the table, nearly between the camera and the picture to be copied-the nearer the object the better, so long as they do not come within the range of the lens.
Exposure. With artificial light the exposure may be anything from two minutes to an hour, depending entirely upon the light employed, while in daylight, out-of-doors, even with a small "stop" it may not be more than a few seconds, and indoors from eight to sixty seconds and perhaps more, depending upon the illumination as seen on the ground-glass.
541. The two greatest difficulties in this work occur in correctly judging the exposure and properly developing the negative. You must remember that however clear and bright may be the subject that you are copying, its contrasts are very much less than those of nature; therefore, if you do not desire a flat result, it is very essential to give a full exposure and follow this up with a full development.
Plate To Use. The plate to use is not so important-any good plate will do for copying (a slow emulsion preferred)-and unless you have a great deal of this work to do it is better to use ordinary plates and to learn to use them well than to undertake to try specially prepared plates, for the average worker will produce better results with the regular plate and the majority of professional photographers use nothing else. One point is important: You must never under-expose; aim to over rather than underexpose and develop to a full strength.
Plates. While ordinary and slow plates are best for general copying, very good originals are improved sometimes by using an Isochromatic plate. Pictures that are old and stained should never be copied with any color-sensitive plate (Isochromatic or Orthochromatic), as these plates tend to show the blemishes much more strongly, being sensitive to the different colors which are found in old pictures. By the use of slow Orthochromatic plates you can make better copies from a fine print, one full of half-tones and color-value, but never use them on old or faded yellow and marred prints, for these plates are sensitive to the different color values and the defects will appear more prominent in the copy than they were in the original. It is advisable to use a slow plate for the reason that slow plates have more latitude both in the exposure and development. A slow plate when over-exposed can be handled much more easily in correcting the exposure during development. There is also less danger of over-exposing a slow plate. Generally speaking, the slower the plate the finer the grain of the emulsion. For the use of special prepared plates and special copying, see Chapters XX and XXI.