A Manual Of Photography

ON THE THEORY OF THE DAGUERREOTYPE. 141 appearance which accompanies excess of bromine ; but, on the other hand, by augmenting the quantity of iodine, the sensitive- ness of the plate is diminished. These difficulties were over- come 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. Tlie 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 cldorate is simply as a solvent of iodine. In tlie subsequent ex- periments, 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, whieli 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 condi- tion; in other words, that its sensitiveness to light is restored. In order to determine this point, tlie following experiments were made.F “ A prepared plate was exposed to light, and afterwards to the mixed vapour ;1 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 the 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 tlie 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 1 “ I shall hereafter call the mixed vapours of iodine and bromine pro- duced in the way described in tlie last paragraph but one, mixed vapour, in order to avoid circumlocution. —Œ S.”