* " I shall hereafter call the mixed vapours of iodine and bromine produced in the way described in the last paragraph but one, mixed vapour, in order to avoid circumlocution.— G. S".
" From these experiments, then, it was perfectly clear that the impression produced by the light on a Daguerreotype plate is wholly destroyed by the mixed vapour, and that its sensitiveness to light is restored.
" It now remained to discover to what extent the sensitiveness is restored by the treatment in question. It was not at first expected that the sensitiveness to light was as great after this treatment as after the original preparation of the plate; but experiments afterwards proved that the surface lost none of its sensitiveness by this treatment, nor even by numerous repetitions of it. A prepared plate was exposed to light ; the impression was destroyed and sensitiveness restored by the mixed vapour; the plate was a second time exposed to light and a second time to bromine; still its sensitiveness appeared unimpaired, for a fourth or fifth exposure gave, on treatment with mercurial vapour, a vivid impression. In order to determine with the greatest accuracy if the sensitiveness of the prepared surface was at all impaired by these repeated exposures to light, the camera obscura was resorted to. A series of plates was prepared with the utmost attention to uniformity ; some of these were exposed in the camera obscura, and pictures obtained by the subsequent exposure to vapour of mercury : the time requisite for the proper development of the picture was noted ; others were first exposed to the direct rays of the sun, and afterwards to the mixed vapour, and these were exposed in the camera obscura for the same length of time as those which had not been exposed to light. On treatment with mercurial vapour, perfect pictures were produced, which could not be distinguished from those taken on plates prepared by the ordinary method. So completely does the mixed vapour restore the sensitiveness of prepared plates after exposure to light, that the most beautiful impressions were obtained in the camera obscura in two seconds on plates which had previously been four times exposed to the direct light of the sun, and after each such exposure treated with the mixed vapour.
"As the plates experimented on, to this stage of the inquiry, had been wholly exposed to the sun's light previous to exposure in the camera obscura, it was thought that possibly some slight effect was produced, which, from being the same on all parts of the plates, escaped observation ; and in order to avoid the possibility of error from this cause, the impressions of light which it was intended to destroy by bromine were afterwards made in the camera obscura. Prepared plates were impressed with virtual images of different kinds, the camera obscura being pointed first at a house, afterwards to a bust, next to a tree, and finally to a living figure, the plates after each impression, excepting the last, being momentarily exposed to the mixed vapour. In every instance, the most perfect impressions of the objects to which the camera obscura was last directed were obtained, and no trace of the previous impressions was left.
" Experiments were next instituted for the purpose of ascertaining if the prepared surface, after the process of mercuriali-zation, could be made to receive another impression by treatment with mixed vapour. Impressions were taken with the camera obscura, and after the full development of the picture by vapour of mercury, the plates were exposed to bromine and again placed in the camera obscura, the instrument being directed in different experiments to different objects: on exposure to mercurial vapour, other pictures made their appearance, and although confused from superposition on the first pictures, could be clearly traced, and were found perfect in every part. This production of picture upon picture was repeated, until, by the confusion of the superposed images, the effects of further exposure could be no longer distinguished.
" In all the experiments hitherto described, the destruction of the impressions by bromine was effected in the dark, the apparatus being situated in a room into which only a very feeble daylight was admitted. It remained to be discovered if the mixed vapour had the power of destroying the effect of light while the plate was still exposed to light, or if the vapour had the power of suspending or preventing the action of light on a daguerreotype plate. In order to determine this point, the apparatus was placed near the window of a well-lighted room, and so arranged that, during the whole time of the preparation of the plate, by exposure first to iodine and afterwards to bromine, it was exposed to full daylight, and by a mechanical arrangement, of too obvious a nature to render description necessary, the plate was withdrawn from the bromine vessel into a dark box; that is to say, it was withdrawn at the same moment from the influence of both light and bromine: on being placed in the camera obscura, plates so prepared received impressions which by mercurialization produced excellent pictures, and there was no trace of the action of any light save that of the camera obscura. It follows, then, that light is incapable of exerting any appreciable influence on daguerreotype plates during the time they are receiving their coatings of iodine and bromine.
" Although these experiments afford no information on the subject in reference to which they were originally undertaken, they are yet not without interest, both in their theoretical bearing and in their practical application. They demonstrate not only that the change (whatever it may be) effected by light on silver plates prepared by Daguerre's process, is completely suspended in the presence of the vapour of either iodine or bromine, but that after that change has been produced the impression may be destroyed, and the plate restored to its original condition, by a momentary exposure to either of these vapours. In their practical application, these experiments show that all the care which has been taken to exclude light from daguerreo-type plates during their preparation is unnecessary; that so far from a dark room being essential to the operations of the daguerreotype artist, the light of day may be allowed to fall on the plate during the whole time of its preparation; and that it is only necessary to withdraw it at the same moment from the action of bromine and light by sliding it from the bromine vessel into the dark box in which it is carried to the camera obscura ; and where, from the situation or otherwise, there is a difficulty in observing the colour of the plate during the process of iodizing, it may be removed from the iodine vessel, and its colour examined by the direct light of the sun, without risk or injury : for when returned to the iodine or bromine vessel for a moment, the effect of light is wholly destroyed.