PHOTOGRAPHIC PRINTING AND ENGRAVING. 3 
printing-press, the plate would be found sadly deficient. A 
careful examination will show where the fault lies. Where 
the lig’ht has acted strongly, the plate is not etched at all ; 
where the light has not acted, the plate will be corroded very 
deeply ; and if this portion represent fine lines, such as the 
branches of a tree or a row of palisades, the ink will be held 
by them, and produce a good print ; but if a surface of the 
plate be etched in this manner, there will be no means of 
holding the ink, and that portion will not, therefore, give an 
impression. Again, a half- tint will be represented on the 
plate by a uniform corrosion of the surface to a slight depth ; 
but for printing purposes, half-tints of various degrees are 
required to be represented by lines or dots of different dis- 
tances apart. 
This difficulty besets all processes for photographic engrav- 
ing ; pure black and white can be given easily enough, but 
the half-tints, which constitute nine-tenths of a good photo- 
graph, have puzzled many experimenters to master. Talbot 
partially overcomes this difficulty by producing an artificial 
aquatint ground on the plate, either by impressing it with 
the image of two thicknesses of black lace crossed diagonally, 
or by spreading very evenly over the surface of the plate 
a little finely-powdered gum copal, and then heating it. In this 
manner the ink is enabled to adhere to those portions which 
constitute the half-tones of a picture ; and by adopting either 
of these artifices, the photoglyphic process, as Mr. Talbot 
terms it, has yielded results which, in the hands of a skilled 
operator, and on small plates, can scarcely be surpassed. 
The photogalvanographic process of Pretsch is somewhat 
similar in its commencement to the . one just described. A 
plate of glass, or other smooth surface, is coated with bichro- 
mate of potash and gelatine, and afterwards exposed to the 
light under a photograph or an engraving ; it is then moistened 
with water, but not thoroughly washed. The first action of 
moisture is to cause those portions of the surface which have 
not been exposed to the light to swell and rise up, more or 
less, in ridges from the surface of the plate. A mould is then 
taken of the plate so raised ; from that an electrotyped copper- 
plate is procured, which is used as a matrix, from which other 
plates may be produced suitable for printing purposes. The 
gelatine in swelling is found to split up into ridges, giving to 
the whole surface a granular effect, which holds the printing 
ink equally well in the fine lines and the broad masses of 
shadow. This process gives very effective prints when they 
are large and are viewed from a distance, but for fine, delicate 
work it is not so successful. 
Another process has been brought to considerable per- 
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