PHOTO-GALVANOGRAPHIC PROCESS.—A photo-mechanical process now no longer used, although many of the present secret processes are based upon it. Pretsch's photo-galvanographic process may thus be briefly described. A glass plate was first coated over with gelatine containing potassium dichromate or silver salt and other chemicals. When dry it was exposed under a transparency. After printing it was immersed in cold water to dissolve out the unaltered chromium salt and to cause those parts protected from the light to swell up in proportion to the protection which had been afforded by the transparency. Further, the raised portions were rough and re ticulated, the reticulation being coarse or fine in proproportion as the different parts had been protected. The high lights did not, of course, swell at all, and had no grain. The plate thus produced was, of course, an exact reverse of what is required for printing. All that was then necessary was to take an electrotype cast from this relief to obtain the printing plate.
Pretsch produced plates of this description about the year 1868.
The Fox-Talbot process was contemporary with Pretsch and some what similar.
Neither process was a commercial success, probably owing to its not being in a sufficiently perfect state to render it independent of the help of the engraver for retouching the plates.
PHOTOOASTROSCOPE.—An arrangement for photographing the inside of the stomach. Fig. 332 illustrates the apparatus. At the end is attached a small electric light which lights up the cavity. An image is transmitted through the lens tube to the camera by means of mirrors. The arrangement has also a fine tube, through which air is forced to inflate the stomach. 'Water is also made to circulate round the electric light.
PHOTOGENE.—To produce.—The generation of a more or less continued impression or picture on the retina, and the delay in the obliteration of it.
This name was also applied by Gaudin to "any sensitive compound containing iodide of silver with excess of free nitrate."