Chemical Force in the Spectrum. 33 
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 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 
im 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 1} times the length of that toG. I 
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 distribu- 
tion in the diffraction and prismatic spectra. 
OF THE CHEMICAL ACTION OF RADIATIONS ON SUBSTANCES. 
eoener 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 absorp- 
tive property of that substance.” 
This involves the conception of selective absorption, as I have 
formerly shown (Phil. Mag., Sept., 1841). A ray which pro- 
uces a maximum effect on one substance may have no effect 
1st.— Of the decomposition of silver iodide. 
There are two forms in which the silver iodide has been used 
for photographic urposes: Ist, when prepared by the action 
of the vapor of iodine on metallic silver, as in the daguerreotype 
tablet; 2d, when nitrate of silver is decomposed by iodide of 
potassium, or other metallic iodides. These preparations differ 
strikingly in their actinic behavior, the former furnishing by far 
the most interesting series of facts. 
Am. Jour. Sc1.—Tuep Serres, Vou. V, No. 25.—Jan., 1873. 
3 
