to prevent any undue separation of the sensitive silver salts. When commencing to make the emulsion, take of hard gelatine 15 to 20 parts. Allow this to swell in distilled water, and as soon as the emulsion has been digested pour off the water from the gelatine, allowing it to drain slightly, and carefully let it melt in the water bath, which it should readily do, as in the course of an hour or so, which has passed in the above operations, it will have absorbed enough water to melt. This melted gelatine should then be added to the emulsion and the whole well shaken, then the froth should be allowed to subside a little, and the emulsion poured out into a flat glass or porcelain dish to set. The emulsion should be about three-quarters of an inch thick in the dish, and should be placed in absolute darkness to set, which takes, as a rule, about five to eight hours, according to the temperature. In the height of summer a little ice placed in the same box as the emulsion will be of assistance; in cold weather this is not necessary. The above process is suitable for plates of medium sensitiveness, and it is just as well to state here that very rapid plates are extremely difficult to make, and fog and all such incidental and minor troubles, such as frilling, blisters, spots, etc., are more likely to make their appearance when trying to make rapid plates. The simplest formula which the writer has tried, and the one in which success is more likely to attend a novice's efforts, is the following of Eder's, which gives a very good, clean working plate of about 150 W.: -
Ammonium bromide ... ...... 20 parts.
Solution of potassium iodide (10 percent.) 3 ,,
Hard gelatine ............ 45 ..
Distilled water ............ 300 ,,
Allow the gelatine to soak in the water for one hour ; dissolve by the aid of a gentle heat; add the salts.
Silver nitrate ... ...... "... 30 parts.
Distilled water ............300 ,,
Dissolve the silver, and convert into the ammonia-nitrate by the addition of ammonia as previously directed. Heat solution A to 35o C. (= 94o F.), and add solution B; digest for 30