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.
Developing Under-Exposures. In case the image appears slowly and with contrast, you will readily understand that the plate is undertimed. Then immediately place it in a tray of plain water. Cover the tray and allow the plate to remain for, say ten minutes; then conclude developing in normal developer. If the plate shows signs of extreme under-timing, make a new developer, weak in pyro (using only one-half the regular quantity), the regular amount of sulphite and carbonate of soda and double the amount of water. Conclude the developing in this bath instead of with normal developer. The resulting plate will be clear in detail. with no harsh high-lights.
283. When Is a Plate Developed? - The greatest difficulty in developing a plate is to know just when to stop. Conditions have so much to do with the proper developing of a plate, that it is hard to state exactly how to tell, under all circumstances, when to stop developing. A properly timed plate is completely developed, and carried far enough, when the contrast between the high-lights and shadows is as you would desire it, and as they appeared on the ground-glass, taking into consideration that the plate loses some of its strength in fixing.
284. Some brands of plates fix out more than others. For instance, Seed plates will lose about two degrees in the hypo bath, while others will lose about one shade of density; therefore, when judging if the plate is sufficiently dense, you must bear in mind the brand of plate and develop accordingly. In examining an over or under-timed plate, you must judge it for the same results as one properly timed, taking into consideration whatever effect the over-timing or irregular conditions may have upon it. Should the plate be slightly over-timed it will thicken up more rapidly, but must be carried further in the developer until the desired contrast, even in a very dense plate, is visible. If the plate has an even density throughout, when fixed, while it may be strong, it will lack the necessary contrast. So it must be carried far enough to produce this result, and stopped there.
285. After looking through your plate, if you are in doubt as to whether the proper density has been obtained, by looking at the back for the image, you can see how far through the film the developer has penetrated. If, upon examination, you find the proper contrast, and the image is fairly clear on the back, your plate is sufficiently developed.
286. Do not depend entirely by the appearance on the back, for in some cases plates will be fully developed before the image appears on the back at all, according to the thickness of the emulsion, which varies in different brands, and also to the length of exposure. In a plate that is slightly under-timed, necessitating weak, prolonged development, which enables the developer to penetrate the film more deeply, the image will appear more distinctly on the back than in a normal exposure in which the action of the developer is more on the surface and the image shows very dimly, or not at all, even when completely developed.
287. You will notice that the high-lights show through first, and if in looking through the plate the shadows appear weak, continue development until they show on the back.
In the case of a double-coated plate, fully timed, the highlights at least should show on the back; if not, the coating being double, the appearance when looking through the negative to the ruby light will be more dense than it really is, and if developed only as far as an ordinary plate it would be very thin when fixed.