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.
When To Stop Development. It requires considerable practice to become expert in judging when a negative is correctly developed. Negatives of various kinds of lightings require different treatment, yet, there is one principle that holds good in all cases. The development must be carried until the highest point of light (the darkest point in the negative) is perfectly opaque (black in the negative). It must not be carried further, as the delicate half-tones, almost as strong as the highest point of light, will then develop down and become just as dense as the high-light - causing a flat, chalky effect in the finished print. The highest point of light must be developed until every particle of silver has been reduced - using a photographic term "Develop until the highest point of light is through to the glass, but stop there, as you cannot drive it into the glass."
Judging Density. Fixing the plate reduces its density to quite an extent - the amount depending on the thickness of the emulsion, which varies in different makes and brands of plates. There are numerous methods employed by photographers to judge density. We recommend the beginner to hold the plate before the ruby light and place one finger close to the film near the strongest high-light. When this highest light in the negative is as dense as the shadow cast by the finger, you can consider the plate developed to the proper strength. When you think development has been carried far enough try to fix in your mind how the plate appears; notice especially the density and its comparison with the deepest shadows. After "fixing," examine the negative in daylight, by looking through it. If the highest point of light is practically opaque, the development has been correct. If the high-light is thin, the negative is under-developed, while if the high-light is dense and covers quite a little space, it is over-developed. Make a close study of each negative and govern yourself accordingly in developing the next plate. By this practice you will soon learn to secure proper density in your negatives under all conditions.
Pin-Holes And Spots. There are endless ways in which pin-holes and spots are produced on the negative. The best way to avoid trouble from this source is to be clean in all operations. Your dark-room, graduates and trays should be kept clean. Your camera and plate holders should be dusted occasionally. The fixing solution should be filtered, after considerable use, to free it from any sediment. Each plate must be carefully dusted before placing it in the holder and before development. The small round spots which are nearly transparent and have dark, defined edges are caused by air-bells adhering to the surface of the plate when the developing solution is first applied, the air-bells preventing the developing solution from acting upon the emulsion. Having the developer in the tray and dropping the exposed plate into the developer, without sufficiently agitating the solution, will almost always cause air-bells and leave the small transparent spots. Have the developing solution in a graduate and place the exposed plate in a dry tray, then flow the plate with an even sweep of developer from the graduate. This method of application will drive off all air from the plate and allow of an even and uniform action of the developer. Air-bells can also be removed by gently passing a tuft of absorbent cotton, thoroughly saturated with developer, over the surface of the plate immediately after the plate is flowed with the developer.