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.
Judging The Time Of Appearance. This is perhaps the most difficult in factorial development, and can only be successfully accomplished by practice. Watch carefully the negative as the image first appears, as it will grow rapidly. The highlights must be watched carefully, and just as soon as the highlights are clearly visible, and the general outline very slightly visible, that is the proper time of appearance, and the number of seconds from the time the plate enters the developer until that stage is reached, is the key to your accurate developing. Now, if you allow the time of appearance to go too far - in other words, if your image be developed too far before taking the time, the result would be that your negative, when completely developed, would be strong; or, if it were not carried far enough when taking the time of appearance, as for instance, if you judge the time of appearance twenty seconds, and it possibly should have been thirty seconds, this time multiplied by the factor would shorten the necessary time for complete development, and consequently the plate would be under-developed.
Obtaining The Factor. If you study carefully the instruction you should not have any difficulty in obtaining the factor. There are a few things very necessary that must be remembered.
First. The factors for pyro and amidol developers are governed by the strength in grains to the ounce solution. In other words, to dilute a pyro bath changes the factor. Why? Because you at once change the grain strength to the ounce solution.
Second. The factor for all other developers depends entirely upon the developing agent, and not upon the formula employed, and a factor does not alter with strength or dilution, but the time of appearance may be altered. For example, if the time of appearance with a normal me-tol bath was fifteen seconds, and the factor was thirty, that would indicate that in four hundred and fifty seconds the plate would be fully developed. Suppose we dilute the metol with equal amount of water, the time of appearance would be slower, perhaps twenty seconds. The factor is the same, and it will require six hundred seconds to completely develop the plate. In other words, diluting the developer only prolongs the development, but does not change the factor in a metol, or long factor developer.
Controlling The Temperature. It is not absolutely necessary that the temperature be exactly sixty-five degrees. Anywhere from sixty to seventy is safe, but beyond this limit you will experience trouble unless you test each batch of plates for time of appearance, and that is all the difference it makes in the accurate time of complete development. It is advisable in cases of very radical changes of temperature, and especially in very hot climates, that the time of appearance be taken for each batch of plates, and as results are governed entirely by time of appearance, you will experience no trouble, for while a bath at sixty-five degrees Fahr. might develop to-day in six minutes, to-morrow if the temperature of the developing room were sixty, the plate would develop fully in eight minutes. All is governed entirely by time of appearance, and you must work accordingly.