A Manual Of Photography

140 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. “ The same experiments were repeated with iodine, with exactly similar results. “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 lead not been exposed to iodine or bromine, exhibited, by the large quan- tity of mercury which condensed on them, the effects of expo- sure 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 ascer- taining in what condition the prepared plate was left after having been first exposed to light and afterwards exposed to the À apour of bromine or iodine. In these experiments a method of treat- ment 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 re- sults, 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 lias a maximum of sensitiveness when the iodine and bromine are in a certain re- lation 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, be- comes 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 tliis 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 bro- mine, 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 risle of producing the