430 Dr. J. W. Draper on the Distribution of 



1872), respecting the difference of calorific distribution in 

 dispersion and diffraction, ^pply likewise to the chemical force. 

 To be satisfied of this, it is only necessary to compare photogra- 

 phic impressions given by a prism and by a grating. 



I published engravings of such diffraction-photographs in 

 1844. They are referred to in the Philosophical Magazine 

 (June 1845). As they were obtained on silver plates made 

 sensitive by iodine, bromine, and chlorine, they do not extend 

 to the hne 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 the amalgam, 

 copying the surface perfectly. Of the series of spectra, 1 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 diffraction-photographs that had ever 

 been obtained. The wave-lengths assigned were according to 

 Fraunhofer's scale, which represent 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 in- 

 stance continued for a whole hour, the impression beyond H was 

 not more than \\ times 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, therefore, showed very 

 clearly the difference of distribution in the diffraction and dis- 

 persion spectra. 



On the Chemical Action of Radiations on Substances. 



Having offered the foregoing evidence in support of the first 

 proposition considered in this memoir, which was to the effect 



*' That, so far from chemical influences being restricted to the 

 more refrangible rays, every part of the spectrum, visible and in- 

 visible, can give rise to chemical changes, or modify the mole- 

 cular arrangement of bodies,^^ I now pass to the second, which is 



" That the ray effective in producing chemical or molecular 

 changes in any special substance is determined by the absorptive 

 property of that substance.'''' 



This involves the conception of selective absorption, as 1 have 

 formerly shown (Phil. Mag. Sept. 1841). A ray which pro- 

 duces a maximum effect in one substance may have no effect on 

 another. Thus the rays which change chlorophyl are not those 

 which change silver iodide. 



