PYRO-SODA AND PYRO-POTASH FACTORS Pyro Bromide Factor Grs. per or. Grs. per or.
.. 9 5 3 • * • • • • 4 4 3+ I o 18 3 .. o 0 8 6+ Factors for soft, normal, and strong contrasts with " tabloid " formulae (Burroughs and Well come) : Soft Normal Strong Amidol . . . 7 ro 12 Edinol . . . 14 20 24 Eikonogen . . 8 12 15 Glycine . . . 9 13 r 6 Hydroquinone. • 3 5 Metol . . . 20 30 35 Metol-hydroquinone. io 14 Paramidophenol . 12 16 i8 Pyro . . 4 6 7 Pyro-metol . 6 9 The factor for a combined developer with the developing agents in equal quantities is the mean of the two, for example : Pero 6 Metol 3o (6 + 3o) 2 = 18If the agents are not in equal proportions the factor for each is multiplied by the number of parts and the results added together and divided by the total number of parts of both agents; thus, supposing that the proportions were pyro 4 parts and metol 2 parts, it would be 6 x 4 = 3o X 2 = 6o 6o + 24 = 84 6 = 14, the factor required.
The above factors are given merely as guides, and those that have been found to give a nega tive of normal contrasts, that is, one with a = but should the worker think that such a negative is too soft or too hard, he has merely to increase or decrease the factor to obtain greater or less contrasts. Supposing one were using a metol-hydroquinone developer with a factor of 15, and the time of appearance were 9 seconds, the total duration of development would be 15 x 9 = 135 seconds.
The great advantage of the factorial system is that it gives the beginner an excellent idea of how long to develop and enables even the advanced worker always to obtain negatives of similar character.