32 THE WONDERFUL CENTURY. CH AP. v. 



present century, when it .was revived by Thomas Young, 

 and was shown by himself, by Fresnel, and other mathe- 

 maticians, to explain all the phenomena of refraction, 

 double-refraction, polarization, diffraction, and interfer- 

 ence, some of which were inexplicable on the Newtonian 

 theory of the emission of material particles, which had 

 previously been almost universally accepted. The com- 

 plete establishment of the undulatory theory of light is 

 a fact of the highest importance, and will take a very 

 high place among the purely scientific discoveries of the 

 century. 



From a more practical point of view, however, noth- 

 ing can surpass in interest and importance the discovery 

 and continuous improvement of the Photographic art, 

 which has now reached such a development that there 

 is hardly any science or any branch of intellectual study 

 that is not indebted to it. A brief sketch of its origin 

 and progress will therefore not be uninteresting. 



The fact that certain salts of silver were darkened by 

 exposure to sunlight was known to the alchemists in the 

 sixteenth century, and this observation forms the rudi- 

 ment from which the whole art has been developed. 

 The application of this fact to the production of pictures 

 belongs, however, wholly to our own time. In the year 

 1802, Wedgewood described a mode of copying paint- 

 ings on glass by exposure to light, but neither he nor Sir 

 Humphrey Davy could find any means of rendering the 

 copies permanent. This was first effected in 1814 by 

 M. Niepce of Chalons, but no important results were ob- 

 tained till 1839, when Daguerre perfected the beautiful 

 process known as the Daguerrotype. Permanent por- 

 traits were taken by him on silvered plates, and they were 



