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.
448. By factorial development is meant the technical method of development in which the process of developing is regulated solely by mathematical figuring, when certain conditions exist. It is the object of this instruction to thoroughly consider these conditions, and to make the subject of factorial development so simple that even a person who has had little or no experience in negative making can take the exposed plate and having received the instructions necessary for proceeding to develop by the factorial method, be able to produce an excellently developed negative.
The Factor. The factor is the agent or the means by which is found the required length of time that the exposed plate must remain in a developing solution composed of a certain developing agent. Originally the factor is found by developing a normally exposed plate in the usual manner, regardless of the nature of developing agent, and in this case, of course, it is necessary, in order to obtain an accurate factor, that the person who develops the plate has a perfect knowledge of the requirements of an ideal negative.
Obtaining The Factor. In order to obtain the factor for developing, after carefully preparing the bath according to the formula, see that the temperature is sixty - five degrees Fahr.; then place a normally exposed plate in the bath, and observe the time required for the strongest highlights of the image to appear fully with the general outlines, also faintly visible. The rapidity of this appearance indicates the speed of the developer. The relation between this time of appearance and the total time to develop is the factor, and all future baths made according to the same formula, and of the same temperature, will completely develop plates in the same time. To make this more clear, before placing the normally exposed plate in the normal developer note the time by your watch or clock, then place your plate in the developer, watch the action of the solution upon the plate, and as soon as the highlights appear fully and a general outline is slightly visible, again note the time, and see exactly how many seconds were required for development to this stage; or, in other words, how many seconds the plate was in the developer. Make a note of this number of seconds, and of course the negative continues developing, and when the plate is completely developed again note the total number of seconds (or minutes reduced to seconds), required for development from the placing of the plate in the developer until it is completely developed. Now divide the total number of seconds by the number of seconds recorded for the first appearance of the highlights, and this result is your factor.
451. When the factor is once found by the method above described, that factor will always hold good in case of a developing solution mixed according to the particular formula used in the test. If another developing agent is employed it is necessary to make another test, and find a factor for that particular developer. Where we have a combination of different developing agents, such as metol - hydroquinone, metol - pyro, etc., separate factors must be obtained for each agent, and the proportionate amount of each used in the formula must be considered in computing the factor for the combination formula. As we supply the Watkins' table of factors of practically all the developing agents, it will not be necessary that you make this test unless you have a particular formula which does not come under the accompanying table. Should that be the case, you will be able to obtain a factor by following the directions given above.