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.
Plate Developing Flat. In portraiture this is due to flat lighting, or over-exposure. If the plate develops up very flat and your subject was properly lighted, and the plate properly exposed, it is because you have diluted your developer. Use a normal, full strength developer. If your plate is over-exposed, treat as such in development, and you will overcome this flatness, and produce more contrast. (See Instruction, Developing Over - Exposures.)
No Detail In Highlights. This is due either to contrasty lighting, or over-development. Watch your plate carefully during development, and make the changes necessary in the developer to control detail in highlights. If the plate is over-exposed, add bromide, or place the plate in old developer. If it is a contrasty lighting, plate exposed about right, reduce the strength of the developer, by adding water.
355. Oftentimes it is a good plan, in this case, to place the plate directly into a tray of fresh water, and allow it to remain five or ten minutes, and then return the plate to a developer which should be only about half as strong as the developer you had been using, and you can reduce the strength by adding water. If the lighting and exposure are correct, and the highlights clog up so that there is no detail, this will indicate that your developer is too strong. Immediately reduce it by doubling the quantity of water. One should always strive to have the lightings correct, and never depend on altering the lightings in the developing.
Retaining Soft Detail In Shadow. In case of contrasty lightings developing hard, and you are unable to produce the desired softness by manipulation, you should over-develop and after thoroughly fixing the negative reduce the entire plate until the highlights appear sufficiently soft, with sufficient strength, however, to give snap and brilliancy. The after reducing will also apply to plates that were properly lighted, but over-developed. By being careful not to allow your highlights to build up too strong, you will produce soft detail in the shadows. Too strong a developer will produce strong highlights and transparent shadows. Dilute the developer in order to secure softness. Under-exposure would produce strong highlights, and deep shadows. Dilute the developer with water. Over-exposure would produce dense highlights and flat foggy shadows. Over - develop and reduce with red prussiate.