448 Mr. G. Shaw on some Photographic Phccnomena. 



of bromine employed was produced by adding thirty drops of 

 a saturated solution of bromine in water to an ounce of water; 

 the solution was poured into a glass vessel, and the plate was 

 exposed to the vapour in this vessel during the time specified. 

 The plates were then introduced into the mercury box, and 

 by volatilizing metal, the pictures were developed on all those 

 which had not been exposed to the vapour of bromine, while 

 those which had been exposed to it exhibited no trace of a 

 picture under ihe action of mercury. 



The same experiments were repeated with iodine, with ex- 

 actly 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 ca- 

 mera obscura. Some of these plates were exposed to the va- 

 pour 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 mer- 

 cury, those plates which had not been exposed to iodine or 

 bromine exhibited, by the large quantity of mercury which 

 condensed on them, the eifects 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 experiments demon- 

 strated that the eifect 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 vapour of bromine or iodine. In these experiments a 

 method of treatment somewhat different from and more con- 

 venient 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 deter- 

 mine with certainty how far some of the appearances pro- 

 duced were due to the action of light. It has already been 

 stated, that a prepared plate has a maximum sensitiveness 

 when the iodine and bromine are in a certain relation to each 

 other ; if there be a deficiency of bromine, the maximum sen- 

 sitiveness is not obtained, and, if there be an excess, the plate 

 is no longer sensitive to light; but when exposed to the va- 

 pour 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 



