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10 on being rendered insoluble by the action of light in parts, imprisons the coloring matter, and thus forms the dark parts of the image. A sheet of paper coated with such a substance mixed with a pigment, exposed to light under a negative, and then washed to remove all the soluble matter, will produce a picture, the blacks of which are formed by the insoluble substance and pigment, and the whites by the surface of the paper from which the colored coating has been washed away.

It will be seen, however, that in the production of an image by means of a photographic negative, in coloring matter so imprisoned, there is no provision for the rendering of gradation of tint. A layer of substance capable of being rendered insoluble by the action of light, if extended on a sheet of paper and exposed to light under a stencil plate, would be rendered insoluble wherever the luminous action penetrated the apertures in the plate. If the paper were then treated with a solvent of the substance with which it was coated, the coating would be removed from all portions protected from the action of light by the opaque parts of the plate, and a perfect transcript of the design would be formed on a white ground. If, instead of the stencil plate, a photographic negative be employed, the image in which is formed by varying gradations of opacity, the result is somewhat different. The layer of soluble matter is rendered insoluble wherever the light has penetrated sufficiently through the transparent parts of the negative; but where the more opaque parts of the negative, through which light has penetrated with much less intensity, protect the coated surface, a portion only of the coating is rendered insoluble, that portion being the surface in immediate contact with the negative. When the prepared paper is submitted to the action of a solvent, the thoroughly-exposed portions, being quite insoluble, are not