Several other preparations have been employed, with variable success, and recommended for procuring an absorbent film upon glass plates—amongst others, the serum of milk has been used by M. Blanquart Everard ; others combine with the albumen or gelatine, grape sugar and honey ; the object of these being to quicken the process, which they appear to do in virtue of their power of precipitating the metals from their solutions.
Blanquart Everard has lately communicated the following to the Paris Academy of Sciences, as an instantaneous process :— " Fluoride of potassium, added to iodide of potassium, in the preparation of the negative proof, produces instantaneous images on exposure in the camera. To assure myself of the extreme sensibility of the fluoride, I have made some experiments on the slowest preparations employed in photography—that of plates of glass covered with albumen and iodide, requiring exposure of at least sixty times longer than the same preparation on paper. On adding the fluoride to the albumen and iodide, and substituting for the washing of the glass in distilled water after treatment with the aceto-nitrate of silver, washing in fluoride of potassium the image immediately on exposure in the camera obscura, I have indeed obtained this result (but under conditions less powerful in their action) without the addition of the fluoride to the albumen, and by the immersion only of the glass plate in a bath of fluoride after its passage through the aceto-nitrate of silver. This property of the fluorides is calculated to give very valuable results, and will probably cause, in this branch of photographic art, a change equally as radical as that effected by the use of bromine on the iodized silver plates of Daguerre." A process published in the authors Researches on Light, in 1844, and named the Fluorotype, sufficiently establishes my claim to priority in the use of the fluorides.
Messrs. Ross and Thompson, of Edinburgh, have been eminently successful operators with the albumen process. Many of their pictures, which are of large size, exhibiting more artistic effect than is produced by any other photographers. Some of the positives produced are very fine. At the meeting of the British Association in that city, these gentlemen exhibited some positive images on glass plates : these were backed up with plaster of Paris, for the purpose of exalting the effects, which were exceedingly delicate and beautiful.
Messrs. Langenheim, of Philadelphia, have, however, recently introduced into this country specimens, which they term Hyalo-types. These are positive pictures, copied on glass from negatives, obtained upon the same material. Their peculiarity is the adaptation of them for magic-lantern sliders. The process by which they are produced is not published, but judging from the effects obtained, the probability is that a very slight variation only from the processes described has been made. The idea is an exceedingly happy one, as by magnifying those images which are of the utmost delicacy and the strictest fidelity, perfect reflexes of nature are obtained.
There can be no doubt but other means of coating glass with sensitive materials may be employed. Certainly the use of albumen is a ready method, but this medium appears to interfere with the sensibility which it is so desirable to obtain. As stated, by using combinations of iodine and fluorine salts, there is no doubt but the sensibility may be most materially improved, and we find many of the continental photographers using honey and grape sugar with much advantage.
I would, however, venture to suggest that films of silver precipitated from the solution of the nitrate by grape sugar, aldehyde, or gun-cotton dissolved in caustic alkali, upon which any change could be afterwards produced, appear to promise many important advantages.