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hyposulphite of soda. To obtain the positive picture, paper must be used brushed over with an ammonio-nitrate of silver, made thus: forty grains of nitrate of silver is to be dissolved in one ounce of distilled water, and liquid ammonia cautiously added till it re-dissolves the precipitate. A pleasing effect may be given to calotype, or indeed to all photographic pictures, by waxing them at the back, and mounting them on white paper, or if colored paper be used, various beautiful tones of color are produced. POSITIVE CALOTYPE. At a meeting of the British Association, Professor Grove described a process by which positive calotype pictures could be directly obtained; and thus the necessity to transfer by which the imperfections of the paper are shown, and which is moreover a troublesome and tedious process, is avoided. As light favors most chemical actions, Mr. Grove was led to believe that a paper darkened by the sun (which darkening is supposed to result from the precipitation of silver) might be bleached by using a solvent which would not attack the silver in the dark, but would do so in the light. The plan found to be the most successful is as follows: ordinary calotype paper is darkened till it assumes a deep brown color, almost amounting to black; it is then redipped into the ordinary solution of iodide of potassium, and dried. When required for use it is drawn over dilute nitric acid-- one part acid to two and a half parts water. In this state, those parts exposed to the light are rapidly bleached,