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186 THE ADVANCE OF PHOTOGRAPHY
time which elapses between pouring the developer upon the plate, and the first appearance of the image, bears a certain definite relation to the total time required for the particular developer in use. This automatically rectifies, to a certain extent, the influence of the temperature of the developer upon the speed of development.
There are, however, drawbacks to this method. In the first place a light is required in order to be able to note the first appearance of the image, and this is not possible with some of the orthochromatic plates ; then, again, the personal equation comes in, for some persons are able to distinguish an image relatively much earlier than others, and so, of course, the calculated time of full exposure will in some cases be incorrect. In some cases, too, this system has been found to fail for temperatures between 45° F. and 60° F.
The following examples of factors taken from Wat-kins' table will help to make the method understood : —
Metol-hydroquinone 14 Eikinogen • . .9 Pyro-soda . 4-15
Glycin soda . . 8
Suppose that the image can be first seen in 20 seconds when using metol, then, since the factor for metol is 30, the total time of development to obtain a negative of average vigour will be 20x30 sec. = 10 min. The higher the temperature the more rapidly will the development take place, and each particular developer has its own " temperature coefficient " — i.e. the factor by which the time of development must be increased or decreased, by a decrease or increase of 18° F. or 10° C. in the temperature from the normal 60° F.
The Time Thermometer. — Watkins has quite recently brought out a simple instrument for overcoming this temperature difficulty.
Hydroquinone
. 5
Ortol
. 10
Rodinal .
. 30
Metol .
. 30