Whichever of these two sensitizers is used, take 1½ drachms, and add to every ounce of the collodion. Collodion thus prepared is _most_ rapid in its action, giving a deep negative (with Ross's sixteen guinea lens, and the developing agent I shall hereafter describe) in ten seconds in clear weather, and instantaneous positive pictures, which may be afterwards darkened with the solution of terchloride of gold, in chloride of ammonium. It does not easily solarize, and, what is best of all, gives the most pleasing half-tones. I find it preferable, in taking landscapes, to rather increase the quantity of the iodide of ammonium, in order to give complete opacity to the sky; but if the operator pleases, he may produce the most admirable effect with the above-named proportions, by painting in clouds at the back of the plate with Indian ink: and this latter plan is preferable, as the addition of more of the iodide lowers the half-tones. {253} If more of the chloride than above specified be added, it will cause the plate to blacken all over during development, before the extreme lights are fully brought up. My developing agent is made as follows. Take Distilled water 10 oz. Pyrogallic acid 6 grs. Formic acid 1 oz. The latter is not to be the concentrated acid, but merely the commercial strength. These, when mixed, form so powerful a developing agent, that the picture is brought out in its full intensity, almost instantly, while at the same time all the deep shades are quite unaffected, and the half-tones come out with a brilliancy I have never seen before. Another excellent developing agent is composed as follows. Take Distilled water 10 oz. Sulphuric acid 3 drops. Protosulphate of iron ½ oz. Formic acid 1 oz. The formic acid is also a most capital addition to the protonitrate of iron, and either this or the former liquid produce most brilliant positives leaving a fine coating of white dead silver. I may also make mention of the improvement I have made in the albumen paper, which consists in the introduction of the chloride of barium into the albumen, in place of chloride of ammonium or chloride of sodium. Take Water 6 oz. Albumen 6 oz. Chloride of barium 7¼ dr.