290 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



obtained : " Place a picture on some paper coated with chloride of 

 silver, and expose the whole to the sunlight, the picture uppermost. 

 The dark portions will stop the rays ; the corresponding parts of the 

 coating, those touched arid covered by these black portions, will 

 retain their primitive whiteness. On the contrary, where the paper 

 on which the picture is printed has retained its semi-transparency, 

 the solar light will pass and blacken the silver layer. The result 

 will be a copy like the original in form, but with a reversal of 

 all the tints ; the lights will be found dark, and vice versa!' 

 Unfortunately these negatives, like Charles's sketches, were not 

 permanent, because the light, continuing to act on the parts not 

 attacked at first, eventually covered the paper coated with chloride 

 with one uniform tint of black. In 1802, Wedgwood succeeded in 

 copying engravings, and in reproducing, on white leather and on 

 paper coated with chloride or nitrate of silver, the- designs on the 

 painted windows of churches ; but he did not think it possible to 

 apply his process to the reproduction of the images produced in the 

 camera obscura. At the same time Sir H. Davy succeeded in 

 obtaining pictures of minute objects by placing them at a short dis- 

 tance from the lens of the solar microscope. These attempts were 

 however incomplete, in the sense that neither Wedgwood nor Davy 

 discovered the means of fixing the images obtained, that is, of pre- 

 venting their disappearance in sunlight. About twelve years later, 

 Mcephore Niepce of Chalons-sur-Saone, who devoted his leisure to 

 scientific studies, also attacked this problem of the photogenic repro- 

 duction of the images seen in the camera obscura. After numerous 

 unsuccessful efforts, he was obliged to give up the attempt to obtain 

 views from nature, monuments, or scenery, on account of the great 

 length of time required by the materials he used to receive the 

 action of light. Until 1829, the time of his association with 

 Daguerre, Niepce confined himself to the photographic copying of 

 engravings ; but he had the satisfaction of succeeding completely in 

 fixing the images, a problem unsolved by Charles, Wedgwood, and Sir 

 H. Davy. We will describe his process in a few words. 



To a sheet of copper, covered with silver and perfectly polished, he 

 applied, with the aid of a stopple, a varnish composed of bitumen dis- 

 solved in oil of lavender. The plate, after being gently warmed, was 

 then found to be covered uniformly with a whitish layer of bitumen 



