A HISTORY OF SCIENCE 



To render the utility of the spectroscope complete, 

 however, it was necessary to link with it another new 

 chemical agency namely, photography. This now 

 familiar process is based on the property of light to de- 

 compose certain unstable compounds of silver, and thus 

 alter their chemical composition. Davy and Wedg- 

 wood barely escaped the discovery of the value of the 

 photographic method early in the nineteenth century. 

 Their successors quite overlooked it until about 1826, 

 when Louis J. M. Daguerre, the French chemist, took 

 the matter in hand, and after many years of experimen- 

 tation brought it to relative perfection in 1839, in 

 which year the famous daguerreotype first brought the 

 matter to popular attention. In the same year Mr. 

 Fox Talbot read a paper on the subject before the 

 Royal Society, and soon afterwards the efforts of Her- 

 schel and numerous other natural philosophers con- 

 tributed to the advancement of the new method. 



In 1843 Dr. John W. Draper, the famous English- 

 American chemist and physiologist, showed that by 

 photography the Fraunhofer lines in the solar spectrum 

 might be mapped with absolute accuracy ; also proving 

 that the silvered film revealed many lines invisible to 

 the unaided eye. The value of this method of observa- 

 tion was recognized at once, and, as soon as the spec- 

 troscope was perfected, the photographic method, in 

 conjunction with its use, became invaluable to the 

 chemist. By this means comparisons of spectra may 

 be made with a degree of accuracy not othenvise ob- 

 tainable; and, in case of the stars, whole clusters of 

 spectra may be placed on record at a single observation. 



As the examination of the sun and stars proceeded, 



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