|From The Journal of the Photographic Society of London, Vol. 1, No. 9 (21 September 1854) pp. 105-106.
To the Secretary of the Photographic Society.
"A New mode of conducting the Daguerreotype Process,"
IN the course of numerous experiments in the Daguerreotype art, I found that after plates were prepared in the usual way with the vapours of iodine and bromine, ready for being impressed in the camera, there was a constant evaporation from their surfaces, to a greater or less extent; and this fact was most marked when I had prepared plates for views, &c., but which could not be used till several hours afterwards: in the latter case, the bromine, more volatile than the iodine, had evaporated more rapidly, and consequently the proportions between the twoso necessary for a good picturewere entirely destroyed. Another circumstance that struck me was, that if a plate were prepared well, and allowed to remain in the dark box a few minutes ere placing it in the Camera, the resulting impression was always more sharply and better defined than if the plate had been immediately used; in the latter case, the impression being not unfrequently somewhat clouded in detail. These observations led me to the conclusion, that when under the ordinary process a plate was exposed in the camera, and the actinic rays allowed to act on its sensitive surface, the evaporation (above alluded to) being let loose in the camera, the vapours again attacked those portions of the surface affected by the actinic agency, and so prolonged the operation; for it is well known that the mixed vapours, so long as they exist, will, if allowed to fall on the plate, altogether neutralize the effect of the actinic power. I hereupon set about thinking of some plan to remedy the defect, and eventually determined to allow the plate to be acted upon simultaneously by the actinic ray and the vapour of mercury or light transmitted by means of a yellow medium, and the result quite bore out my expectation.
| I have now constructed a camera which simplifies the operation much, and which for a travelling Daguerreotypist is a great improvement, as it does away with the necessity of a mercury box, and saves all the time and trouble of changing the plate from one box to another, pouring and repouring the mercury, &c. It is formed thus: A, fig. 1, is the usual form of camera, showing a back view. B is a cast-iron bottle of the capacity (for mercury) of say 3 or 4 ounces, and screwed in a convenient place, on about the centre of the hinder or sliding part of the camera; and C is a close-fitting cock, to shut off or allow to escape the mercurial fumes when heated. Fig. 2, D, is a pipe, say of metal, bent from the upper part close to the side of the camera, to allow of its being conveniently heated with a spirit lamp.|
| I have found the same results effected by admitting light into the camera through a yellow medium; but as this can only be done in a a strong light, it would not be of such general use as the preceding plan.|
I may mention here, that I am one of those who hold that it is the colour of the vapour (yellow) of mercury which developes the latent picture on the plate, and not particles of mercury attaching themselves to the silver, as some affirm. The sun's rays, through a yellow or orange medium, act precisely in the same manner; but I cannot now enter upon my reasons, as it would occupy too much space and too much of your valuable time. With regard to the process that I have now communicated, it may be suggested that the mercurial vapour would injure, or, at all events, not improve the back lens, upon which, of course, it must of necessity fall in cameras constructed upon the present plan, though hitherto I have not found my lens affected at all; but to obviate any risk of such damage, the camera should have an air-tight partition of clear blue glass immediately in rear of the lens.
|W. H. STANLEY CRAWFORD.|
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