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 must be pure white, and the surfaces flat and parallel. After cleaning it in the ordinary manner, I cover it with alcohol 30 grammes, water 8 grammes, and nitric acid 10 drops. I dry it, and throw on the surface a small quantity of dry soap powder, the excess of which I remove by gentle touches of a fine brush.

2d. Coating with Collodion.—My collodion consists of ether 150 grammes, alcohol 80 grammes, and gun cotton 6 grammes, and the glass is covered with it in the usual manner. After drying, I proceed to

3d. Application of the Sensitized Gelatine.—This preparation consists of

To this is added a sufficient quantity of some aniline color; for instance, Magenta, to give a warm tone to the picture.

This preparation is poured over the dry collodion film, and dried in a horizontal position, so that the film attains the same thickness throughout.

4th. Exposure in the Solar Camera.—The printing is done from behind. The side not coated is exposed to the image of the negative, and the time of this exposure is only half as long as that required to obtain a print on chloride of silver paper.

5th. Development.—This is simply done by repeated washings in warm water, and, when the soluble gelatine is removed, washing is continued with cold water; then the glass is dried in an oblique position.