The only method I know of securing a uniform quality of gelatine is to purchase several small samples, make a trial emulsion with each, and buy a stock of the sample which gives the best results. To those who do not care to go to this trouble, equal quantities of Nelson's No. 1 and X opaque, as recommended by Captain Abney, can be employed. Having selected the gelatine, 1¼ ounces should be allowed to soak in water, and then melted, when it will be found to have a bulk of about 6 ounces. In order to prepare our emulsion, I take equal bulks of the silver nitrate and potassium bromide solutions in beakers, and place them in the water bath to get hot. I also take an equal bulk of hot water in a large beaker, and add to it one-half an ounce of the gelatine solution to every 12 ounces of water. Having raised all these to about 180° F., I add (as you observe) to the large beaker containing the dilute gelatine a little of the bromide, then, through a funnel having a fine orifice, a little of the silver, swirling the liquid round during the operation; then again some bromide and silver, and so on until all is added. When this is completed, a little of the emulsion is poured on a glass plate, and examined by transmitted light; if the mixing be efficient, the light will appear--as it does here--of an orange or orange red color. It will be observed that we keep the bromide in excess while mixing. I must not forget to mention that to those experienced in mixing, by far the best method is that described by Captain Abney in his Cantor lectures, of keeping the silver in excess. The emulsion, being properly mixed, has now to be placed in the water bath, and kept at the boiling point for forty-five minutes. As, obviously, I cannot keep you waiting while this is done, I propose to divide our emulsion into two portions, allowing one portion to stew, and to proceed with the next operation with the remainder. Supposing, then, this emulsion has been boiled, it is placed in cold water to cool. While it is cooling, let us consider for a moment what takes place during the boiling. It is found that during this time the emulsion undergoes two remarkable changes: 1. The molecules of silver bromide gradually aggregate together, forming larger and larger particles. 2. The emulsion increases rapidly in sensitiveness. Now what is the cause, in the first place, of this aggregation of molecules: and, in the second place, of the increase of sensitiveness? We know that the two invariably go together, so that we are right in concluding that the same cause produces both. It might be thought that heat is the cause, but the same changes take place more slowly in the cold, so we can only say that heat accelerates the action, and hence must conclude that the prime cause is one of the materials in the emulsion itself.