To Sir John Herschel we are indebted for the first use of glass plates to receive sensitive photographic films.

The interest which attaches to this is so great, and there appear to be in the process recommended by the English experimentalist so many suggestive points, from which future photographists may start, that the passages are given in Sir John Herschel's own words:—

"With a view to ascertain how far organic matter is indispensable 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 lay 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 supposed to have emanated from the hempen filaments, which were barely in contact with the edge of the glass, and which were constantly abstracting matter from its surface in place of introducing new.

" Exposed in this state to the focus of a camera with the glass towards the incident light, it became impressed with a remarkably well-defined negative picture, which was direct, or reversed, according as looked at from the front or the back. On pouring over this cautiously, by means of a pipette, a solution of hyposulphite of soda, the picture disappeared, but this was only while wet; for on washing in pure water and drying, it was restored, and assumed much the air of a daguerreotype when laid on a black ground, and still more so when smoked at the back, the silvered portions reflecting most light, so that its characters had, in fact, changed from negative to positive. From such a picture (of course before smoking) I have found it practicable to take photographic copies; and although I did not, in fact, succeed in attempting to thicken the film of silver, by connecting it, under a weak solution of that metal, with the reducing pole of a voltaic pile, the attempt afforded distinct indications of its practicability with patience and perseverance, as here and there, over some small portions of the surface, the lights had assumed a full metallic brilliancy under this process. I would only mention further, to those who may think this experiment worth repeating, that all my attempts to secure a good result by drying the nitrate in the film of chloride have failed, the crystallization of the salt disturbing the uniformity of the coating. To obtain delicate pictures the plate must be exposed wet, and when withdrawn must immediately be plunged into water. The nitrate being thus abstracted, the plate may then be dried, in which state it is half fixed, and it is then ready for the hyposulphite. Such details of manipulation may appear minute, but they cannot be dispensed with in practice, and cost a great deal of time and trouble to discover.

" This mode of coating glass with films of precipitated argentine or other compounds, affords, it may be observed, the only effectual means of studying their habitudes on exposure to light, free from the powerful and ever-varying influence of the size in paper, and other materials used in its manufacture, and estimating their degree of sensibility and other particulars of their deportment under the influence of reagents. I find, for example, that glass so coated with the iodide of silver is much more sensitive than if similarly covered with the chloride, and that if both be washed with one and the same solution of nitrate, there is no comparison in respect of this valuable quality; the iodide being far superior, and of course to be adopted in preference for the use of the camera. It is, however, more difficult to fix, the action of the hyposulphites on this compound of silver being comparatively slow and feeble.

" When the glass is coated with the bromide of silver, the action, per se, is very slow, and the discoloration ultimately produced far short of blackness ; but when moistened with nitrate of silver, sp. gr. 1.1, it is still more rapid than with the iodide, turning quite black in the course of a very few seconds' exposure to sunshine. Plates of glass thus coated may be easily preserved for the use of the camera, and have the advantage of being ready at a moment's notice, requiring nothing but a wash over with the nitrate of silver, which may be delayed until the image is actually thrown on the plate, and adjusted to the correct focus with all deliberation. The sensitive wash being then applied with a soft flat camel-hair brush, the box may be closed and the picture impressed, after which it only requires to be thrown into water, and dried in the dark, to be rendered comparatively insensible, and may be finally fixed with hyposulphite of soda, which must be applied hot, its solvent power on the bromide being even less than on the iodide".

Sir John Herschel suggested a trial of the fluoride of silver upon glass, which, he says, if proved to be decomposable by light, might possible effect an etching on the glass, by the corroding property of the hydrofluoric acid.

The metallic fluorides have been found to be decomposable, and a very sensitive process on paper, called the fluorotype, will be described in the chapter on Miscellaneous Processes. I am not aware that any experiments have been made directly upon glass, but it is certainly worthy of a careful trial.

Herschel has remarked that we cannot allow the wash of nitrate to dry upon the coating of the chloride or iodide of silver. If, however, we dip a glass which has one film of chloride upon it into a solution of common salt, and then spread upon it some nitrate of silver, we may very materially thicken the coating, and thus produce more intense effects. Mr. Towson employed glass plates prepared in this manner with much success. The mode adopted by that gentleman was to have a box the exact size of the glass plate, in the bottom of which was a small hole; the glass was placed over the bottom, and the mixed solution, just strong enough to be milky, of the salt and silver poured in. As the fluid finds its way slowly around the edges of the glass, it filters out; the peculiar surface action of the solid glass plate, probably a modified form of cohesive force, separating the fine precipitate, which is left behind on the surface of the plate. By this means the operation of coating the glass is much quickened. Another method by which films of any of the salts of silver can be produced upon glass plates, is the following modification of the patent processes of Drayton and of Thompson for silvering glass:—

Take a very clear plate of glass, and having put around it an edging of wax about half-an-inch in depth, pour into it a solution of nitrate of silver made alkaline by a few drops of ammonia, taking care that no oxide of silver is precipitated ; mix with this a small quantity of spirits of wine, and then add a mix-ture of the oils of lavender and cassia, or, which is perhaps the best process, a solution of grape sugar. In a short time the glass will be covered with a very beautiful metallic coating. The solution is now poured off, the edging of wax removed, and the silver is exposed to the action of diluted chlorine, or to the vapour of iodine or bromine, until it is converted into a compound of one of these elements, after which we may proceed as recommended by Sir John Herschel.