DEVELOPMENT, FACTORIAL A system of determining the duration of development by noting the time of appearance of the first trace of an image on the plate and multiplying this time by a factor, the result being the total duration of development required to produce a negative of given density. This method was suggested by Alfred Watkins in 1893, and has been found in practice to be very reliable except in a few cases of exceptionally low temperatures and very dilute developers. It may be looked upon as one of the first prac tical steps to reduce development from mere happy-go-lucky guesswork to a definite and exact method. Like all methods based on laws it is elastic and capable of adjustment to the individual worker's ideas of what is a correct negative ; or, in other words, by reducing or increasing the factor a thinner or denser nega tive may be obtained suited to the particular printing process employed. It is an ingenious application of Hurter and Driffield's law of con stant density ratios, and is based on the fact that with correct exposure the total duration of development for a given density bears a fixed ratio to the time of appearance of the image, assuming that the developing power of the solution remains constant, and this rule holds good for variations in strength of the developing agent, in the amount of the alkali, bromide, and temperature.
The following factors are those generally used for the principal developers :— Adurol • • 5 . . . • 3o Certinal . . . . 3o Cristoid pyrocatech'n . . 3o Diogen . . . I 2 Edinol Zikonogen . . • . 9 Glycine-potash . . . 12 Glycine-soda . . . . 8 Hydroquinone bromide . 5 Imogen sulphite . . . 6 Kachin . . . . io Kodak powder . . .
Mequin . . . . 12 Metol . . . . . 3o Metol-hydroquinone . • Ortol . . . . . io Paramidophenol . . .
Pyrocatechin . . .
Pyro-metol (Imperial Standard) 9 Pyro-soda . . . . 4-15 Quinomet . . . . 3o Rodinal . . . . 3o Synthol .