prints show only ghostly high lights ; longer development improves the shadows and gradations ; very long develop ment still further levels matters, till in the end the shadows are so filled up that printing becomes well-nigh impossible.
Temperature.—The normal temperature for solutions is 65° Fahr. In hot weather developers should be used much weaker than the usual strength, and in cold weather some means of keeping the dark room warm is very desirable. Some developers—quinol and ortol especially—will work very slowly at a low temperature, and at 55° action practically ceases. But it is a mistake to warm the developer unless the fixing bath and washing water can also be raised to the same temperature.
Fixing Bath.—When development is adjudged complete the plate should be washed in three changes of water, and may then (except in hot weather, or when the gelatine edges show much frilling) be transferred direct to the fixing bath consisting of hyposulphite of soda and water in the pro portions of 4 ozs. to the pint. It is an old-fashioned practice to add a few drops of ammonia to the hypo bath. After 5 minutes' immersion in the hypo bath the negative may generally be examined by daylight without serious injury, but fixation will not be complete for at least io minutes, or about 3 minutes after all traces of the white bromide of silver have disappeared at the back of the film. The same hypo solution can be used again and again for fixing plates until it becomes discoloured or fixes too slowly ; the bottom of the dish must be frequently sponged to remove the black precipitate which collects there. Large firms and clubs collect their stale hypo in a barrel, the contents of which are periodically evaporated, and the residues sent to a refiner, who recovers the silver.
Acid Fixing Bath.—Many workers prefer to the above alkaline bath Prof. Eder's acid bath, which is considered to give greater clearness and brightness. Mix tartaric acid solution (1 to 2) ?, oz. with sodium sulphite solution (I to 4) 11 oz., and then add the mixture to each pint of the ordinary hypo bath. Another very good one is made by adding oz. of potassium metabisulphite to each pint of hypo solution.
Washing the Negative.—On removal from the fixing bath the plate is well rinsed under a tap or in several changes of water, and must then lie either in a washing tank or in running water for several hours to remove the last traces of hypo from the film. Hypo left in the film will not only lead to spots, stains, and iv:emature fading of the negative, but may injuriously affect the printing papers placed in contact with -it. Or, after an hour's washing, it may be placed in a clearing solution consisting of alum i oz., citric acid oz. (or nitric acid 20 drops), water to oz. Five minutes will suffice for this before a final washing under a tap, when the plate may be put away in a rack or some other secure place to dry.
Hypo Eliminators.—To obviate the prolonged washing of the negative several preparations have been suggested. Per oxide of hydrogen in a very weak solution, r dr. to io oz. of water, is one of these, and in such proportions will not injure the image very seriously. Another is percarbonate of potash 25 gr., water to oz. Easier than these, as reliable, is to wash the negative (after a rinse under the tap) in five changes of washing water, to each of which just enough permanganate of potash solution or Condy's Fluid has been added to turn the liquid a faint pink. The caution must be given that all hypo eliminators are strong oxydisers, and the greatest care is necessary that they only reach the film after proper mixing, and in the very dilute form recommended.
Drying Negatives rapidly.—That negative has the best chance of turning out satisfactory which is allowed to dry naturally at an even and moderate temperature. But if prints are wanted in a hurry, blot off superfluous moisture with hard hairless blotting paper, or dab with cotton wool, and then flood with good quality methylated spirit. Five minutes will be sufficient for the plate to remain in the spirit. A safer method is five minutes in the formaline bath, the usual proportions being formaline I oz., water 20 oz. Aftcr wards dip in water for a minute, drain off, and if necessary dry near the fire or a foot above a gas flame, keeping it in motion by a swinging movement of the arm.