The Progress of Photography. 



Photography may be described as the Art (or Science) 

 of reproducing the images of objects by the agency of Hght. 

 Of course it has been known from time immemorial that 

 light has the power of altering or wholly discharging the 

 colours of certain substances; but it was not until theyear 1802 

 that Thomas Wedgewood availed himself of the facility with 

 which nitrate of silver, combined with some organic sub- 

 stance, is darkened by the agency of sunlight, to make 

 experiments in the copying of pictures by this means, and 

 about the same time profiles thrown on the wall were 

 taken at Paris, in the same way by M. Charles, on paper 

 overspread with chloride of silver. You know, of course, that 

 if a diaphanous or partially transparent body, as a piece of 

 lace, or a leaf, be laid upon such a surface, those portions of 

 the prepared paper which are least shaded by the object are 

 darkened in proportion, and we obtain a reversed or 

 " negative " image, in this case a white picture on a dark 

 ground. 



Of course it soon occurred to the experimenter, that if 

 he could thus reproduce shadows or images of objects in the 

 flat, he ought to be able to copy solid objects, nay, to realise 

 a landscape itself with its varied outlines, its gradations of 

 light and shade, as it appears on the screen of the camera 

 obscura. But it was found that the silver chloride which 

 was easily darkened by the sun's direct rays, received little 

 impression after indefinite exposure to the very much weaker 

 light which reaches it through the camera lens. Passing 

 by the experiments of Niepce, I come to the important 

 discovery of Daguerre. Daguerre substituted for silver 

 chloride, spread on paper, a thin film of silver iodide 

 produced by the action of iodide on a silver plate. He 

 did get a picture in the camera but it was a weak 

 one and required a very long exposure. One day, however, he 

 put away in a cupboard a plate which had received a very 

 slight exposure, there being, of course, no visible image upon 



