416 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 



tion in dispersion and diffraction, apply likewise to the 

 chemical force. To be satisfied of this it is only neces- 

 sary to compare photographic, impressions given by a 

 prism and a grating ! 



I published engravings of such photographs in 1844. 

 They are referred to in Memoir VI. As they were ob- 

 tained on silver plates made sensitive by iodine, bro- 

 mine, and chlorine, they do not extend to the line F. 



I had found that certain practical advantages arise 

 from the use of a reflected instead of a transmitted 

 spectrum. The ruled glass was therefore silvered upon 

 its ruled face with tin amalgam, copying the surface per- 

 fectly. Of the series of spectra I used the first. 



The fixed lines were beautifully represented in the 

 photographs. They were, however, so numerous and so 

 delicate that I did not attempt to do more than to mark 

 the prominent ones. These were, I believe, the first dif- 

 fraction photographs that had ever been obtained. The 

 wave-lengths assigned were according to Frauuhofer's 

 scale, which represents parts of a Paris inch. 



The length of the photographic impression given by the 

 prism I was then using, from the line H to the ultra-violet 

 end of the spectrum, was about three times that from H 

 to G ; but in the spectrum by the grating, though the 

 exposure was in one instance continued for a whole hour, 

 the impression beyond H was not more than 1J times 

 the length of that to G. In more moderate exposures 

 the last fixed line in the photograph was about as far 

 from H on one side as G was on the other. This, there- 

 fore, showed very clearly the difference of distribution in 

 the diffraction and prismatic spectra. 



