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SCIOPTICON MANUAL. 63 copied in proper position in front of the lens, with noth- ing to shut off the diffused light between the lens and negative. In selecting a position for operations, a north light will be found the best suited for the purpose. The upper window sash should be lowered, and the board upon which the cameras are arranged rested one end upon a table, the other upon the lowered sash, so that the negative will have the sky for a background. This is easily determined by focusing upon the ground-glass. It is advisable not to allow the direct rays of the sun to illuminate the negative. Should a north light not be obtainable, cover a frame with white tissue-paper, and let that be the background for the negative. The tis- sue-paper can be illuminated with the sun's rays, or by any artificial light. It may seem to the reading photographer, unneces- sary to burden this article with a complete chemical formula for making glass positives, but as it is prepared expressly for the uninitiated, it would be unintelligible without a formula. To make 8-02. Sensitive Collodion. —Alcohol 5 oz., ether 3 oz., iodide of ammonium 44 grs., bromide of magnesi- um 20 grs., cotton (Parys') 35 grs. Before using, filter several times through cotton soaked in alcohol. It is a good plan to keep a supply of plain, unexcited collodion on hand, as a stock-bottle; also, a bottle of exciting solu- tion, made in the proportion of iodide of ammonium 5 grains, bromide of ammonium 2£ grains, to the drachm of alcohol. By adding 1 drachm of the solution to 1 ounce of plain collodion, it will be excited to the proper condition. Nitrate Solution. —Water 1 ounce, nitrate of silver 40 grains; make slightly acid with nitric acid, C. P. Iodize