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.
365. The entire success of this method of development lies in sufficient exposure, and as the latitude as to extreme exposure is so great we must strive only to give full time, as any reasonable amount of over-exposure can be treated in the development of the plate. In the wet plate days we had less difficulty with these obstacles than we have with the dry plate. Why? For the reason that the emulsion of a wet plate was not 1/50 as sensitive to white light as an ordinary dry plate to - day; consequently, there was more latitude to the exposure.
366. Next to the wet plate we have the process plate, or the lantern slide plate, all of which are extremely slow plates. For the lantern slide we require the clearest plate possible. The lantern slide must be absolutely clear and free from fog - shadows must be transparent. With the extremely rapid plate this would be impossible to produce except where the most accurate exposure is given, and even then the results are not as satisfactory nor uniform. While the extremely slow-plate is superior for quality, yet for general commercial work the slow plate would be impracticable. In many instances quite rapid exposures are necessary to obtain certain results which could not be obtained with the slow plate, and therefore the most rapid plate must be employed and a means of producing the same results, as is possible with the slow plate, must be accomplished in the development.
367. As stated in the forepart of this instruction, the entire success of special development rests in the exposure. You must time for the most dense shadows, and time them fully; a little over-time will do them no harm, as the overtime can be cared for in the development. By timing for the most dense shadows with this development the highlights will care for themselves, for you treat them in the developer so as to preserve them.
368. A good guide for exposure would be as follows : - Where you would ordinarily give ten seconds, with this method give from thirty to forty seconds' exposure, etc. All the detail in the most dense shadows must be supplied by the exposure, the rest you obtain in the developing,,
369. The slow process of developing, applied by this method, will, with a full - timed exposure on a fast plate give you the same excellent results as a long exposure would give you on a slow plate in which the action of light upon the plate is so slow that it does not fog the plate. With a slow plate developed in a normal developer all chemicals act equally, for there are no great differences to overcome, while in the fast plate there are enormous differences in the effect of light between the strongest highlights and the most dense shadows. In severe cases, by this method of timing fully the most dense shadows, the highlights would be extremely over timed; therefore, in order to give us a well-balanced negative, we must restrain the highlights during development, and hold them in check until the shadows are fully developed.