BOTANICAL PHOTOGRAPHY 183 



acid solution. Citric acid, sulphuric acid, and oxalic acid are also used 

 as preservatives with pyro. 



Sodium carbonate. — This makes the emulsion swell and thus allows 

 the metol and other developing agents to penetrate more rapidly. The 

 dry granular form is better than the crystal or anhydrous forms. 

 Weigh carefully because too much carbonate will cause fog. Stir vigor- 

 ously until dissolved. 



Potassium carboriate. — Hydrochinon works faster with potassium 

 carbonate than with sodium carbonate, but the potassium carbonate 

 is more likely to cause fog, and it may cause blistering and frilling, 

 especially in warm weather. Sodium carbonate is to be preferred. 



Potassium bromide.- — The bromide retards the action and keeps 

 down fog. It restrains the developing agent from acting upon the sil- 

 ver which has not been affected by light. Too much bromide retards 

 the action so much that details are not likely to be brought out in the 

 shadows. The right amount checks chemical fog and produces con- 

 trast and clearness by retarding the development of the shadows, a 

 desirable result, especially in case of overexposed negatives. 



Hyposulphite of soda. — There is great difference of opinion in regard 

 to the composition of the "hypo" bath. Many prefer plain hypo, 

 about 100 g. hypo to 400 c.c. of water. Professor W. J. G. Land, whose 

 negatives, lantern slides, and paper prints could hardly be surpassed, 

 uses this plain hypo bath, which is also preferred by many of the best 

 English photographers. With this bath, there should be an acid "short- 

 stop" between the developer and the bath. Pour acetic acid into water 

 until it tastes about like a weak vinegar. If too strong, it will cause 

 pimples. If no short-stop is used, the alkaline developer is carried over 

 into the hypo and soon spoils it. About 10 or 15 seconds in the short- 

 stop will usually be long enough. 



The addition of alum to the hypo is desirable for negatives because 

 it hardens the emulsions ; but it is undesirable for lantern slides or pa- 

 per prints if you wish to color them, because the alum makes them 

 hard to color, so that various "sizing" solutions have to be used. 



FORMULAS 



For convenient reference some well-known photographic formulas 

 have been brought together here. There will be a still further saving 

 of time if the formula is pasted on the bottle and coated with shellac to 

 keep it from getting wet. While the scientist has no use for antiquated 



