390. Tentative Development. This method of development is only practicable in the treatment of a small number of negatives ; it is used chiefly with pyrogallol ; as an example of its working, we shall take a case in which caustic soda is used as alkali (Drouet, 1898).
When ready to start work, the following bath is prepared.— Soda sulphite, . 25 to 30 gr. to 2 grin.) anhydrous Caustic soda' (io% solution) 2 to 3 drops I'vrogallol 2. 8 to 9 gr. to 0-6 grm.) Water, to make . 31 oz. (too c.c.) If, after about 30 seconds' immersion in this bath the image has not appeared, pour the liquid into a measure and add two more drops of the caustic soda solution, mix, and pour the mixture again on to the plate or film ; continue these additions if necessary until the high-lights are faintly visible. Once the high-lights are clearly indicated, one or two drops of the caustic soda solution may be added from time to time, and the quantity may be increased a little towards the end of development.
For over-exposed negatives .add to the bath as soon as possible a few drops of a xo per cent solution of potassium bromide. If the negative is known to be over-exposed it is best to do this before commencing to develop. The additions of caustic soda must be made less frequently, and towards the end the quantity of pyrogallol must be increased a little.
For under-exposed negatives, as soon as possible increase the amount of caustic soda solution up to about ro drops. If the negative is known to be under-exposed (e.g. when a negative exposed under identical conditions has already been developed), it is better to omit the pyrogallol altogether at first, this being added 30 or 40 seconds after placing the plate or film in the alkali. Continue adding caustic soda during development.
391. Development in Tropical Climates. Various special precautions must be taken in hot climates. In the first place, development should not be deferred more than a few days, since the latent image sometimes suffers a gradual fading (regression) under the combined influence of high temperature and atmospheric moisture. Films are especially liable to this
fading of the image. Next, the gelatine must be prevented from swelling excessively during the process ; such swelling might lead to various troubles (melting, frilling, reticulation, etc.) Finally, sudden changes of temperature must be avoided with the swollen gelatine ; these only increase the risk of accidents, and it is better to carry out the whole of the operations in baths at the surrounding temperature, even though it be high, than to use chilled bathc for some parts of the process and warm water for others. In equatorial regions it is also advisable to carry out the work when the temperature is not so high, generally during the In order to prevent the swelling of the gelatine without preliminary hardening, addition is made to the developer (and to any baths such as desensitizers used before development) of io per cent to 20 per cent of soda sulphate (§ 356) ; or alcohol is substituted for a certain proportion of the water.
As the result of a systematic study 2 of a great number of developers used at various tempera tures up to 95° F., J. I. Crabtree (1917) recom mended the following developer Soda sulphite, anhydrous oz. (5o grm.) Paraminophenol hydrochloride 6o gr. (7 grm.) Soda carbonate, anhydrous . s oz. (5o grm.) Soda sulphate, crystals . 2 to 4 oz. (so° to 200 grin.) Water, to make . . 20 oz. C.C.) The maximum quantity of soda sulphate is employed only if the developer is at a tern perature of about 95° F. ; at about 8o° F. the minimum amount stated is quite enough. A slight reticulation should be produced only after 4 or 5 minutes' immersion in the developer, a time greatly in excess of the normal duration of development, in spite of the retardation due to the sulphate. Development takes twice as long with the lesser quantity of sulphate stated and three times with the larger quantity.
Although this developer has little tendency to give chemical fog, it may be necessary. especially at the higher temperatures, to add to it a small amount of potassium bromide, unless exposures are uniformly increased.