PHOTOGRAPHY, SCIENTIFIC ASPECTS 



of motion, attempted to show that the changes in chem- 

 icals attributed to the action of light by Scheele were 

 really due to the action of heat ; but he was unsuccessful 

 in these attempts, Scheele's contention being strength- 

 ened rather than weakened by Rumford's arguments. 



The first practical application of Scheele's discovery 

 to picture-reproduction seems to have been made by 

 Thomas Wedgwood, a member of the famous Wedg- 

 wood family, of England, in 1802. His process, which 

 was first described in the Journal of the Royal Institu- 

 tion, was to moisten paper or white leather with a solu- 

 tion of silver nitrate in a dark room, and then expose 

 the moistened surfaces to sunlight. In this manner 

 the colorless moistened portions of the sheets quickly 

 became black if exposed to the full rays of the sun, 

 although they could be kept indefinitely without change 

 of color if screened from the light. This discovery sug- 

 gested the possibility of reproducing pictures, or "shad- 

 owgraphs," as they were called, and Wedgwood pointed 

 out the advantages of such a process in reproducing 

 prints from transparencies on glass. 



Here was the germ of the idea from which practical 

 photography has been evolved. But there were still 

 many intermediate discoveries to be made before even 

 the crudest photographs were possible. At that time 

 there was no means of forming a camera-image, or 

 "negative," the nearest approach to a camera being the 

 camera obscura in which an image was shown upon 

 ground glass. Nevertheless the possibilities of the dis- 

 covery were recognized, and Sir Humphry Davy made 

 some experiments with the camera obscura; but noth- 



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