A Manual Of Photography

CHAPTER VII. PHOTOGRAPHS ON GLASS PLATES, AND REGENT IMPROVEMENTS. To Sir John Herschel we are indebted for the first use of glass plates to receive sensitive photographic films. Section I.—Precipitates of Silver Salts. The interest which attaches to this is so great, and there appear to be in the process recommended by the English experi- mentalist so many suggestive points, from which future photo- graphists may start, that the passages are given in Sir John Herschel’s own words. “With a view to ascertain how far organic matter is indis- pensable to the rapid discoloration of argentine compounds, a process was tried which it may not be amiss to relate, as it issued in a new and very pretty variety of the photographic art. A solution of salt of extreme dilution was mixed with nitrate of silver, so dilute as to form a liquid only slightly milky. This was poured into a somewhat deep vessel, at the bottom of which 1ay horizontally a very clean glass plate. After many days the greater part of the liquid was decanted off with a siphon tube, and the last portions very slowly and cautiously drained away, drop by drop, by a siphon composed of a few* fibres of hemp, laid parallel and moistened without twisting. The glass was not moved till quite dry, and was found coated with a pretty uniform film of chloride of silver, of delicate tenuity and che- mical purity, which adhered with considerable force, and was very little sensitive to light. On dropping on it a solution of nitrate of silver, however, and spreading it over by inclining the plate to and fro (which it bore without discharging the film of chloride) it became highly sensitive, although no organic matter could have been introduced with the nitrate, which was quite pure, nor could any indeed have been present unless it be sup- posed to have emanated from the hempen filaments, which were barely in contact with the edge of the glass, and which were