coloured Films formed by Iodine, fyc. upon Metals. 427 



to the action of light. Instead of this property being ex- 

 clusively confined to a film of iodide of silver, as obtained 

 in the process of M. Daguerre, I found that it existed in 

 many other substances when presented to the action of light 

 in the state of thin films, viz. by the bromide and chloride of 

 silver ; by the oxide, bromide, iodide and chloride of copper 

 and some others ; all these however possessing less sensibility 

 than the iodide of silver of Daguerre, and therefore less avail- 

 able for the reproduction of the images of the camera than 

 the compound originally discovered by that gentleman. The 

 iodide of Daguerre was found already too little sensitive to 

 the influence of light in this climate, especially when applied 

 to the reproduction of the image of animate objects, so that 

 those films discovered by me seemed still less suitable- to be 

 employed for that purpose ; this objection has, however, been 

 completely removed by recent improvements, more particularly 

 those of M. Claudet, who effected this principally by com- 

 bining the original discovery of Daguerre with those men- 

 tioned above as having been subsequently made by myself. 

 Pursuing the first stage of Daguerre's process, he obtained 

 the film of iodide of silver, and, added to this another film of 

 bromide, either in a simple state, — as practised in my experi- 

 ments published more than six months before, — or after two of 

 these substances had been combined together, as the chloride 

 of iodine and the bromide of iodine, which he was the first 

 to employ. 



These coloured films, however, merit attention independ- 

 ently of the purposes to which they may be applied in pho- 

 tography : the beauty of some of the phaenomena themselves 

 is peculiarly attractive ; the numerous changes of colour they 

 undergo, either by a variation in the thickness of the film, or 

 by the action of light, assign them a place among the most 

 curious facts of science, and the extreme facility with which 

 they are obtained adds to the interest they excite. 



Impressed with these ideas, I was induced to pursue a train 

 of investigation on this subject; among the results of which,, 

 one of the most interesting was a new method of making co- 

 loured rings, like those generally known under the name of 

 " Newton's coloured rings," on many of the metals, by the 

 same chemical process as that employed for forming the films 

 of uniform thickness in photography. In order to procure these 

 coloured rings, and at the same time to show the identity of 

 the origin of the colours with those of the ordinary transpa- 

 rent films, that is, as residing simply in the thickness of the 

 lamina and not dependent on the ordinary cause of colour, 

 we have but to place a piece of iodine on a well-polished sur- 



