yellow Rays on Daguerreotype Plates. 203 



the discovery of the accelerating action of bromine and chlo- 

 rine, which we have found to be 100 limes more sensitive*. 



When the plate is prepared with this coating of iodine, the 

 red and yellow glasses destroy tiie effect of daylight produced 

 on that coating, in the same manner as when the iodized plate 

 has been subsequently submitted to the vapour of bromine. 



There are, then, certainly some cases when the red and 

 yellow glasses ai e not endowed witli the property of continuing 

 the action conimenced by daylight on the simply iodized plate, 

 but when they are endowed with the property of destroying 

 that action. This highly sensitive coating of iodine is ob- 

 tained in the following manner:^ 



When a plate of silver is submitted to the vapour of iodine, 

 it assumec at first a yellow tint, and afterwards becomes suc- 

 cessively rose, red, violet, blue and blue-green ; all these 

 various tints constitute what 1 shall call the simple coating ; 

 they are all sensitive nearly in the same degree. 



In continuing to iodize, a second coating is formed in a 

 series of the same tints as the first. The plate becomes yellow 

 a second lime, and it passes successively through the rose, red, 

 violet, blue, and blue-green tints: this second coating is 

 twenty-five times more sensitive than the first. But the most 

 sensitive point is about the rose tint. 



A third coating with the same tints can be obtained by a 

 longer exposure to the vapour of iodine; but it is less sensi- 

 tive than the second ; the surface of the silver begins to be 

 attacked by the strong action of the iodine ; and after the 

 washing with hyposulphite it appears milky, which injures the 

 purity of the image. 



If a silver plate be submitted to the vapour of iodine in such 

 a manner as to give it gradually, by horizontal zones, all the 

 tints of the first and second coatings, and in that state en- 

 tirely affected by daylight, then exposed only on one vertical 

 half during a few minutes under a red giass, so that the action 

 of the red glass be exercised only on one half of each zone of 

 the various tints of the two coatings, the mercurial vapour 

 afftects the surface in such a manner, as to show that the red 

 glass has destroyed the action of daylight on the second coating 

 of iodine, and lias continued the same action on the first. 

 The red glass has brought back the half of the more sensitive 

 coating to the same degree oi effect produced on the less sen- 

 sitive, which has not received the action of the red glass. It 



• 1 made the discovery of the accelerating property of chlorine, bromine 

 and iodine combined in certain proportions in May 1841, and I communi- 

 cated .-i paper on the subject to the Royal Society, which was read tlie 

 10th of June following. 



