X 



PHOTOGRAPHY IN ITS SCIENTIFIC ASPECTS 



IN the development of photography, at least, history 

 has repeated itself under peculiarly similar circum- 

 stances. Two of the most important discoveries 

 in this field, one made by Daguerre early in the nine- 

 teenth century, and the other by Becquerel just at its 

 close, were made quite accidentally, and in practically 

 the same manner. Becquerel discovered radio-activity, 

 and Daguerre discovered a practical method of develop- 

 ing photographic plates, by accidentally leaving photo- 

 graphic material in a dark chamber. 



The fact that certain chemicals quickly change color 

 when exposed to light was known for half a century 

 before practical photography was invented, the Swedish 

 chemist, Karl Wilhelm Scheele, having discovered this 

 about 1780. Curiously enough, it was this same scien- 

 tist who discovered that color could be removed, as well 

 as produced, chemically, both these discoveries being of 

 the greatest commercial importance. In experimenting 

 with the silver salts Scheele found that the color of a 

 solution containing these salts could be changed by 

 rays of ordinary light, or by light passing through blue 

 glass, although the color was not affected by light passing 

 through red or yellow glass. A few years later, Count 

 Rumford, the discoverer of the fact that heat is a form 



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