Emulsion. Photographically, a mechanical mixture of any sensitive salt of silver in extremely minute division, held in suspension in any viscous vehicle, such as gelatine or collodion, which, when spread upon any transparent medium, shall present a generally homogeneous appearance when viewed "by transmitted light. The material upon which the emulsion may be spread may be either glass, paper, or any special substance desired. The emulsion may be either for the production of negatives, transparencies, positives, or lantern slides, for development or printing out. In this article that intended for negative work alone will be treated of. To summarise all the methods which have been suggested for emulsion making would be almost impossible, at least, within reasonable limits. Practically, however, we may consider that there are three methods in general practice, viz. -
(a) The ammonio-nitrate process.
(b) The acid-boiling process.
(c) The cold process.
The ammonio-nitrate process, which may be briefly described as the conversion of the nitrate of silver into the double salt by means of ammonia, and the addition of this to a bromised gelatine solution, and digestion of the emulsion at a moderate temperature for a given period, is simple and easy to carry out, and the one specially suitable for amateur emulsion makers. The acid-boiling process requires more apparatus than the previous one, and though clean working plates are obtained, the sensitive-ness is not quite so great as with ammonia. The cold process is simple and easy, requires no heating, but it is far more difficult to obtain regular results.
(a) The Ammonio-Nitrate Process. Eder was the first to suggest this process, and the following is his latest formula. Two solutions are required.
Potassium bromide...... ,..... 24 parts.
Solution of potassium iodide (io'per cent.) 3 to 8 ..
Hard gelatine (Winterthur) ...... 20 ,,
Distilled water ............250 ,,
Allow the gelatine to soak for from 30 to 60 minutes in the water in a closed vessel, then place the vessel in a water bath and allow the gelatine to dissolve, and add the haloid salts. Now