Mr. G. Shaw on some Photographic Phcenomena. 4-4-7 



sence of a larger quantity produces a state of insensibility to 

 light. In order to determine the absorption of the proper 

 quantity of the vapour of bromine (for bromine, from its supe- 

 riority, is now used to the exclusion of chlorine), a very dilute 

 solution of bromine in water is introduced into a glass vessel, 

 and the iodized plate is suspended over it for a time varying 

 according to the strength of the solution and the temperature, 

 and which can be determined only by experiment. Instead 

 of the treatment described, other processes may be resorted 

 to for the preparation of the plate; a solution of chloride 

 of iodine alone communicates sensitiveness to a silver plate, 

 without a previous exposure to iodine; and instead of pure 

 chlorine or bromine, the chloride of bromine, or bromide of 

 iodine, may be used after the iodine. These, and various 

 other accelerating substances, have been proposed, and are 

 used by different experimenters; but they all contain chlorine 

 or bromine, or both, and differ from each other only in the 

 ingredients in which they (the active agents) are disguised. 

 In the experiments hereafter related, it must be borne in mind, 

 that although iodine and bromine are alone mentioned, and 

 were for the most part used, yet the same results have been 

 obtained by the use of all the accelerating agents enumerated, 

 as well as several others. 



A silver plate prepared by the process described, may be 

 exposed to the vapour of mercury without being in any way 

 affected by the exposure. If however the prepared plate be 

 previously exposed to light, or made to receive the luminous 

 image formed in the camera obscura, the mercurial vapour 

 attacks it; forming, in the former case, a white film, and in 

 the latter, a picture corresponding to the luminous image 

 which had been allowed to fall on it. 



If a prepared plate after receiving a vertical impression by 

 light be exposed to the vapour of iodine or bromine, it is 

 found that the vapour of mercury no longer attacks it ; or, in 

 other words, the impression produced by light is destroyed. 



The first experiments made for the purpose of arriving at 

 the cause of this pheenomenon, had reference to the relation 

 between the time of exposure to light and the time of expo- 

 sure to the vapour of iodine or bromine necessary to destroy 

 the effect produced by light. Prepared plates were exposed 

 in the camera obscura for a length of time, which previous ex- 

 periment had determined to be sufficient for a full development 

 of the picture; some of those plates were exposed during two 

 seconds to an atmosphere feebly charged with the vapour 

 of bromine, while others were carefully preserved irom con- 

 tact with the vapours of iodine or bromine. The atmosphere 



