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.
Weak Ruby Light. Even with the ruby lamp lighted, when you first enter the dark-room difficulty will be experienced in seeing, and you might think that you have not enough light. It requires very little light to work by, but your eyes must first become accustomed to it. Before you start to develop, close your dark-room door and remain for a few minutes until you become accustomed to the semi-darkness. When you are ready to examine your plate, hold it up close to your ruby light and look through it. After a little experience you can work intelligently with the dim light. Care should be taken in using an oil lamp, that the flame is not too high, as it will smoke, causing unnecessary heat and odor. Remember, the strongest attribute of the dry plate is that it is affected by light; therefore, it MUST be handled in the least amount of light possible.
Chemicals Will Not Dissolve. The powders, perhaps, have not been mixed with the water in proper order, or the water is too cold. Always dissolve the sodas first - the largest package, or the larger of the two amounts of chemicals in the tubes. The developing agent dissolves very readily, but you should stir the water rapidly while adding it. In adding the soda, do not pour in too much at one time, as it will cake and become hard, when it is almost impossible to dissolve it. Warm water will break up the crystalline formation much more readily than cold water, and will also hold in solution a greater quantity of the chemical.
No Image Appears Upon Continued Development. The plate is badly under-exposed or not exposed at all. In the latter case you may have forgotten to draw the slide, or your shutter did not work. Possibly the shutter was not set, if it is one that needs setting at each exposure. While making the exposure the focusing cloth may have hung over the lens.
Image Flashes Up And Quickly Darkens When Flowed With Developer. If the plate becomes black when covered with the developer it has been extremely over-exposed, either by the shutter not closing properly, or white light, other than that which came through the lens at the time of exposure, has affected it. In other words the plate has been fogged. If the edges of the plate protected from the light by the edges of the plate holder do not become dark, the trouble is over-exposure. If the plate holder was not placed in the back of the camera properly, the plate could have fogged and the edges of it still remain clear upon development. Or, if the shutter on your lens is loose, especially on box or film cameras, and you happen to hold the camera (even if you are not making an exposure) so that the sun strikes the lens, the light is admitted and causes a fog. To avoid any possibility of fog, the greatest care should be exercised at every stage in the handling of the plate. Be sure that your ruby lamp is perfectly safe. See that no white light enters into the dark-room from any source whatsoever. In placing the plate holder in the camera, see that it is perfectly flush with the back of the camera. (See paragraph No. 92.) Make a record of every exposure, and if the plate develops up quickly and becomes very dark almost immediately - the edges of the plate remaining clear - the exposure for that particular plate was too much.