PHOTOGRAPHY. 3 



in a camera to the action of light controlled by a lens, the 

 part acted strongly upon by light would become insoluble, 

 and thus when a solvent was applied (such as naphtha or 

 petroleum) the shadows would be represented by the metal 

 plate, whilst the lights would be formed of the dark resin. 

 The image in this case would be reversed in the character 

 of its shades, unless the black body could be whitened, 

 and the metal blackened. After many experiments with this 

 object in view, Niepce applied iodine to the image obtained 

 with the asphaltum. These bitumen pictures are still in 

 existence, one or more being in the British Museum at the 

 present time. 



In 1824 Daguerre, who was devoted to painting, com- 

 menced similar experiments to those of Niepce, each working 

 in secrecy and unknown to the other. In 1826, however 

 through the want of reticence of an optician, who was a 

 acquaintance of both, the fact that each of them was workin 

 in the same direction was learnt by the other, and in 1829 

 they entered into a kind of partnership. Here it may be 

 that Daguerre first learnt the treatment of metallic plates 

 with iodine, and watched the action that took place in the 

 light, when silver was employed to receive the layer of 

 asphaltum. At any rate to Daguerre belongs the discovery 

 of the action of light on iodide of silver surfaces, and also 

 the merit of producing a picture in the camera with but a 

 short exposure. When I say short, I mean short compared 

 with that given to the bitumen plates, for with such it took 

 six or eight hours to obtain an image. Working with 

 silver plates, which had been subjected to the vapour of 

 iodine, he succeeded at first in obtaining visible images 

 with prolonged exposure ; but whilst endeavouring to obtain 

 them in a moderate time he waded through endless experi- 

 ments, and only chance befriended him at last. It thus 

 occurred : Having exposed some iodized plates in the camera 

 and obtained no results, he placed them away in a cup- 

 board containing a medley of chemicals. On opening it 

 some time afterwards to procure an old plate to clean for 

 fresh trial he found, much to his astonishment, one of them 

 with a fully developed image upon it. I will not exhaust 

 your patience by detailing how he traced the agency at work 

 which had caused this development. Suffice it to say that 

 it was found to be mercury (which vaporises at ordinary 



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