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.
Dodging In The Printing. For a portrait negative where the entire image is flat, we advise covering the glass side with Prussian-blue water-color. This can be procured from any art store. If any portions of the negative are sufficiently dense and do not need intensifying, with the finger rub off the color from these portions. If you wish to strengthen any particular high-lights in portraits or landscapes after applying the blue, coat these portions with a little yellow gamboge. This latter is a sort of paste and is applied like the former (Prussian-blue water-color), by placing a drop of the paste on the parts to be covered, and with the end of the finger tapping the paste gently it will spread uniformly and will not leave any streaks. Understand, do not rub this paste, but simply tap it with the edge of the finger, as you would tap with a hammer. The grain of the flesh in the finger will produce a soft, stipple effect upon the plate. Any portions you would like to print stronger than others must be softened less.
227. Another way of holding back weak shadows is to cover the entire frame with tissue-paper, and before placing the tissue to be printed on the negative, hold it up to the light, look through the negative and note the portions you wish to strengthen or hold back. Dip the finger into some dry yellow ochre and rub over the tissue-paper on the parts you wish strengthened. This will give you very soft effects, equal to former methods, and is much more simple and less expensive. Any of these methods can be employed. The yellow being more opaque than the blue, care must be exercised that you do not apply it too heavy. Should you desire any odd figures or designs introduced into the background, by the application of the gamboge you can cover the portions where you expect to design your figure, and then with the etching knife or with the blade of any ordinary penknife, you can scrape away the surplus paste applied. Some very catchy designs can be worked in the negatives in this way.
228. As previously stated, after a little experience several negatives can be printed from at the same time, using one actinometer, carefully noting the exact number to which each must be printed; but all frames must be placed in the light at the same time, or as nearly so as possible; otherwise uneven prints will be the result. With a little practice the use of the actinometer becomes much easier, and several negatives can be printed from simultaneously as easily as one can be handled. You can always rely upon your actinometer as being exact, except during dull, damp weather. Then the carbon tissue is a trifle less sensitive, which must be allowed for accordingly in the printing. In extremely bright weather the reverse is the case; therefore, use diffused light in bright weather and you will produce uniform results. In winter the tissue is less sensitive than in summer, in proportion to the sensitiveness of the printing-out paper in the actinometer; therefore, in winter a tint one or two numbers higher should be taken for the guide.