The film which I employ is collodion, and in order to insure success, the collodion must be prepared after my own process, as follows: Take 3 ounces (Troy) of pure dry nitrate potassa, and pulverize in a clean glass mortar; add to this 2-1/2 ounces, fluid measure, of pure sulphuric acid, and stir the mixture with a glass rod; immerse in this liquid, 80 grains of clean, dry cotton, and knead the mass of cotton in the liquid for about Ave minutes; remove the cotton and quickly wash it, till every trace of acid is gone, and it must then be dried quickly--for I have discovered that the more rapidly the cotton is dried, the more sensitive the collodion; and I have found the best effects produced by displacing the water from the cotton by strong alcohol. To prepare the collodion, take 10 ounces concentrated sulphuric ether, 60° Baumé, and mix this with 6 ounces of 95 per cent. alcohol. To this mixture add the prepared cotton, in quantity sufficient to make a collodion as thick as it can, and yet at the same time flow evenly over the surface of glass. Let it settle clear, and decant the solution. In order to "excite" this collodion, take a deep 1 ounce vial--introduce 2-1/2 grains of bromide of potassium, and add water, drop by drop, to make a saturated solution. In this solution dissolve 2-1/2 grains of iodide of potassium, then add 1 ounce of collodion, and shake well. Let it settle clear and decant for use. The solution must be decanted every day. In order to make the most sensitive collodion, I dissolve the bromide and iodide of potassium and the collodion[G] in a saturated solution of carbonate of ammonia in water. In using this collodion, pour it upon a clean glass plate to form a film in the usual way, and as soon as the collodion has set, and before it becomes dry, immerse the plate in a bath of nitrate of silver, made with 30 grains of nitrate of silver, 2 grains of iodide of silver, and 1 ounce water. Take the plate directly from the bath to the camera, and after sufficient exposure, the plate is taken to a dark room to develope the impression with the following solution: 'Take pyrogallic acid, 4 grains; acetic acid, No. 8, 1 ounce; dissolve and filter. For use, take of this liquid 1-1/2 drachms, diluted with 6-1/2 drachms of water, and when the impression is sufficiently developed, pour off the liquid, and immerse the plate in a solution of the hyposulphite of soda, 4 ounces to the pint of water. Wash the plate with pure water, and dry it in the usual way. [Footnote G: This is a mistake: the collodion is not to be dissolved in the "carbonate of ammonia in water," but only the bromide and iodide of potassium. I called Mr. Cutting's attention to this, and he said I was correct.--S. D. H.]