washed away from the upper surface, leaving undisturbed the insoluble layer of various thicknesses, which forms the picture on the paper. This method was found to be comparatively impracticable from the long exposure it rendered necessary; the picture suffered also, and became coarse, from the granular texture of the paper through which the light had to pass. Various methods were tried, the details of which appear in a subsequent chapter; but none were found efficient until Mr. Swan discovered the method of preparing a sensitive film, which, having been exposed to light in direct contact with the negative, could be transferred to another support, so as to permit the easy washing away of the unaltered material at the side opposite to that which had been exposed to light. This method was found not only more simple in practice, but more excellent in result than any method previously attempted. It gave pictures of exquisite delicacy and force, rendering perfectly every gradation in the negative with a degree of beauty which had rarely been obtained even by the usual methods of silver printing.
It is unnecessary to enter into a consideration here of the possibility of producing permanent pictures by the usual photographic processes of silver printing. The stigma of instability has been hitherto the chief drawback to the beautiful results of photography. In no silver printing process has immunity from fading been obtained; and although many photographs have been preserved unchanged for years, others, produced under apparently the same conditions, have become, during the same period, faded and worthless; and uncertainty of permanence, if not certainty of fading, remains a characteristic of all silver prints.
In Swan's carbon process the image is produced in the pigments of the painter, and whatever of permanence
