" Prepared plates were exposed to diffused light in the shade, and others were exposed to the direct rays of the sun ; the object being in both cases the production of a more intense impression than that produced by the feeble light of the camera obscura. Some of these plates were exposed to the vapour of bromine, and others to the vapour of iodine, while others were carefully preserved from the vapours of these substances. On subsequent exposure to the vapour of mercury, those plates which had not been exposed to iodine or bromine, exhibited, by the large quantity of mercury which condensed on them, the effects of exposure to intense light : while those which had been subjected to the action of either bromine or iodine were in no way affected by the vapour of mercury. Many repetitions of these experi-'ments demonstrated that the effect of exposure to the most intense light, was completely destroyed by the shortest exposure to the vapour of bromine or iodine.
" Experiments were now instituted for the purpose of ascertaining in what condition the prepared plate was left, after having been first exposed to light and afterwards exposed to the vapour of bromine or iodine. In these experiments a method of treatment somewhat different from, and more convenient than that described, was resorted to, as in practising that method effects occasionally presented themselves which interfered with the results, and rendered it difficult to determine with certainty, how far some of the appearances produced were due to the action of light. It is well known, that a prepared plate has a maximum of sensitiveness when the iodine and bromine are in a certain relation to each other : if there be a deficiency of bromine, the maximum sensitiveness is not obtained, and, if there be an excess, the plate is no longer sensitive to light; but when exposed to the vapour of mercury, without having been exposed to light, becomes white all over, by the condensation of mercury thereon ; that is to say, it exhibits the appearance of a plate which had been properly prepared, and which had been exposed to light. From this it will be evident, that a plate properly prepared in the first instance, and then exposed to light, may, by subsequent exposure to the vapour of bromine, have the impression produced by light wholly destroyed ; and yet, by the accumulation of bromine, may exhibit, on exposure to mercury, an appearance similar to that due to light. In other words, it is impossible (in the case supposed) to distinguish between an effect produced by light and an effect due to excess of bromine. By using iodine in the place of bromine, there is no risk of producing the appearance which accompanies excess of bromine; but, on the other hand, by augmenting the quantity of iodine, the sensitiveness of the plate is diminished. These difficulties were overcome by using a solution containing both iodine and bromine, in such proportions that the evaporation of each should take place in the proportion in which they produce on silver the most sensitive surface. The solution employed was made by adding alcoholic solution of iodine to a solution of chlorate of potash, until the latter would take up no more of the former ; and to each ounce, by measure, of this solution, ten drops of a saturated solution of bromine in water were added. The solution of chlorate of potash was made by diluting one part of a saturated solution of the salt with ten parts of water. The use of the chlorate is simply as a solvent of iodine. In the subsequent experiments, the plate was exposed to the vapour of this mixture of iodine and bromine with precisely the same effect as when either was used separately, and without the inconvenience, or uncertainty, which attended their use.
" A number of preliminary experiments, the detail of which would be uninteresting, appeared to indicate, that not only is the effect of light on a daguerreotype plate destroyed by iodine or bromine, but that the plate is restored to its original condition ; in other words, that its sensitiveness to light is restored. In order to determine this point, the following experiments were made. " A prepared plate was exposed to light, and afterwards to the mixed vapour ; * mercurial vapour produced no effect upon it after a long exposure ; the plate on removal from the mercury box was a second time exposed to light, and again introduced into mercurial vapour. The appearance of this plate was very little changed, and it was concluded that no effect, or, if any, very little, was produced by the second exposure to light. This conclusion was, however, erroneous, as the following experiments proved:—
" A prepared plate was exposed to light, and afterwards to the mixed vapour : mercurial vapour was found to have no effect upon it; the plate was then partly covered with a metallic screen, fixed close to, but not in contact with it, and the whole was exposed to light. On placing the plate in the mercury box, a broad white band, nearly corresponding to the edge of the defended part, made its appearance ; the whole of the defended part (excepting the band in question) was unaffected, and the exposed part exhibited very little change. By a careful examination of the plate after it was removed from the mercury box, the white band in the middle appeared to be produced by the feeble light which had passed under the edge of the metal plate which had screened the light from part of the prepared surface; and the very dark, and apparently unaltered appearance of the exposed part, was occasioned by an excess of action, for mercury was found to have condensed on that part in large quantities, and to have produced the dark lead colour which is commonly called solarisation; but which effect, in the case in question, was so excessive, that the colour of the part on which mercury had condensed differed but very slightly from that on which no light had fallen. It was now evident that the apparent absence of effect in the last experiment was in reality occasioned by an excess of action; and by repeating that experiment, and making the time of the second exposure to light much shorter than before, the plate assumed, under the action of mercury, an intense and beautiful whiteness.