SIR J. F. W. HERSCHEL ON PHOTOGRAPHIC PROCESSES. 
3 
in the experiments described in Arts. 205, 208, in the latter of which a chemical 
action, distinct from the calorific, seems also traceable. I may here also mention 
that the rays which operate the change of colour in muriate of cobalt from rose colour 
to green appear to be the calorific rays generally, and the effect to be one of simple 
evaporation ; since under the action of the spectrum I find the green colour not re- 
stricted to the “ parathermic” region, but to extend far beyond the red, and to be, in 
fact, commensurate with the calorific spectrum, so far as it could be traced in an ex- 
periment made under unfavourable circumstances. 
I have the honour to remain, my dear Sir, 
Yours very truly, 
J. F. W. Herschel. 
Collingwood, Nov. 15, 1842, 
