202 PHOTOGRAPHIC FACTS AND FORMULAS 
Fix 10 minutes, wash 1 to 2 hours, and do not dry between 
blotters. 
HAMMER OPAL PLATES.— 
Metol 5g Y4 oz. 
Hydrochinon 20 g 1 oz. 
Sodium sulphite 75g 334 02. 
Sodium carbonate 125 ¢ 6% oz. 
Potassium bromide 1:25 to'2.5/ Y% to 1 dr. 
Water 1000 ccm 50 oz. 
Add from 120 to 200ccm (6 to 100z.) wood alcohol to 
prevent precipitation. For use mix 1 part of above with 6 
parts water. 
FACTORIAL DEVELOPMENT FOR BROMIDE PAPER.—Dr. B. J. 
Glover puts forward a strong case for the application of the 
Watkin’s factorial system for bromide papers, which, how- 
ever, is not applicable to development papers on account of 
the very short time of appearance. The developer he adopts 
is the Kodak amidol formula at a temperature of 17° C., and 
he finds a remarkable coincidence between the exposure multi- 
plied by the time of development, in seconds, for prints 
showing practically the same result. This forms a constant, 
which is 1,800. Three cases are cited in which the exposures 
were 30, 15 and 10 seconds respectively and the factors 10, 
15 and 22¥% used. The results were practically indistinguish- 
able ; and the total times of development were 60, 120 and 180 
seconds respectively, which, as will be seen, when multiplied 
by the exposures, equal 1,800. He formulates his first rule 
as follows: development must not be for a shorter time than 
that required to produce the maximum black of the paper. 
Accepting this, it will be obvious that it will be unnecessary to 
calculate both exposure and development; one only need be 
determined and the other is at once deduced therefrom, which 
practically forms his scond rule. As he puts it: a knowledge 
