Reduced to its simplest form, the making of a sensitive emulsion has a good deal of resemblance to cooking. Solutions of the reacting salts are weighed out and mixed, one of them containing a little gelatin that acts as a protective colloid and as means of preventing sedimentation of the halide precipitate. Any amateur cook knows the difficulty of making a good mayonnaise or hollandaise sauce. Some of the tricks of the emulsion-maker are comparable to those of the adept culinary artist, and the sensitometric characteristics of the finished emulsion largely depend on the exact method of mixing and cooking.