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CHAPTER VII. 



INVISIBLE PHOSPHORESCENCE. 



I HAVE given the name of Invisible Phosphores- 

 cence to some curious phenomena discovered by 

 my ingenious friend M. Niepce de St. Victor, 

 who communicated them to me with much kind- 

 ness before they were published. During latter 

 years he has, however, addressed to the Academy 

 of Sciences, at Paris, a number of notes and papers, 

 in which his experiments are detailed.* The basis 

 of them all was the following interesting obser- 

 vation : 



M. Niepce discovered that if an engraving be 

 exposed for some time to the sunlight, and then 

 taken into a dark room, and placed upon a sheet 

 of photographic paper prepared with chloride of 

 silver, an impression of the engraving is produced 

 in a very short space of time upon the paper. This 

 experiment was immediately tried with a great 

 variety of substances, such as white porcelain with 

 * ' Comptes-Rendus' from 1857 to the present time. 



