That which was the chief aid of Niepce in the humblest dawn of the
art, viz. to transform the photographic plate into a surface capable
of being printed, is in the above process done by the coöperation of
Electricity with Photography. This invention of M. Pretsch, of Vienna,
differs from all other attempts for the same purpose in not operating
upon the photographic tablet itself, and by discarding the usual means
of varnishes and bitings-in. The process is simply this: A glass tablet
is coated with gelatine diluted till it forms a jelly, and containing
bi-chromate of potash, nitrate of silver, and iodide of potassium. Upon
this, when dry, is placed face downwards a paper positive, through
which the light, being allowed to fall, leaves upon the gelatine a
representation of the print. It is then soaked in water; and while
the parts acted upon by the light are comparatively unaffected by the
fluid, the remainder of the jelly swells, and rising above the general
surface, gives a picture in relief, resembling an ordinary engraving
upon wood. Of this intaglio a cast is now taken in gutta-percha, to
which the electro process in copper being applied, a plate or matrix is
produced, bearing on it an exact repetition of the original positive
picture. All that now remains to be done is to repeat the electro
process; and the result is a copper-plate in the necessary relievo, of
which it has been said nature furnished the materials and science the
artist, the inferior workman being only needed to roll it through the
press.--_Quarterly Review_, No. 202.