speaking of the application of a mixture of gelatine, bichromate, and pigment to paper, he says:
“It must be observed that the possibility of producing half-tones by this plan rests on the power of the insolubility—causing actinism to penetrate, with a certain degree of facility, the mixture of pigment with bichromate and gelatine, or gum, the gelatine or gum being in consequence rendered insoluble, to a greater or less depth on different parts of the picture, according to the varying depth to which the actinism has been allowed, or had time, to penetrate; this, again, being dependent on the varying translucency of the different parts of the negative.”[1]
It was at this juncture of the history of carbon printing, we learn, that Mr. Swan commenced his experiments. Without being aware of Mr. Burnett's suggestions, and prior to the publication of Mr. Blair's letter (presently to be mentioned), he attempted to obtain half-tone by coating a plate of glass with a mixture of lamp-black, solution of gum arabic, and solution of bichromate of potash. After drying the coated plate, he exposed it in the camera, with the uncoated surface of the glass turned towards the light passing through the negative and lens. The plate was then washed in water, with the view of removing, from the back of the sensitive coating, those portions which the light had not rendered insoluble. The experiment was not successful (probably in consequence of the too feeble action of the light); it shows, however, that Mr. Swan not only, ab initio, recognized the true principle upon which the production of half-tone in carbon-printing depends, but, furthermore, applied it in a very elegant manner.
Early in the following year, Mr. Blair, of Perth, made the same discovery;[2] for it is worthy of note that in
