420 CHEMICAL FORCE IN THE SPECTRUM. [MEMOIR XXIX. 



ible modification or chemical decomposition sets in. The 

 film begins to darken ; it passes through successive tints 

 brown, red, olive, blue and eventually becomes dark 

 gray. 



I have described in some of the foregoing paragraphs 

 the action of the spectrum on silver iodide as presented 

 on the tablet of the daguerreotype, showing the differ- 

 ence in the impression obtained : 1st, when extraneous 

 light has been excluded ; and, 2d, when it has been per- 

 mitted simultaneously or previously to act. 



In the latter case, in all that region of the spectrum 

 from the more refrangible extremity to somewhat below 

 the line G, the usual darkening effect manifested by silver 

 compounds is observed ; but beyond this, and to the ex- 

 treme less refrangible rays, with certain variations of in- 

 tensity, the action of the extraneous and simultaneously 

 acting light is checked, and the effect of previously act- 

 ing light is destroyed. 



It happened that in 1842 I obtained two very fine 

 specimens of the latter spectra; one of these I sent to 

 Sir J. Herschel, the other is still in my possession. 



In the Philosophical Magazine (Feb., 1843), Herschel 

 gave a detailed description of these spectrum impressions. 

 He was disposed to refer the appearance they present to 

 the phenomena of thin films, but at the same time point- 

 ed out the difficulties in the way of that explanation. 

 He also sent me three proofs he had obtained on ordi- 

 nary sensitive paper, darkened by exposure to light, then 

 washed with a solution of iodide of potassium, and placed 

 in the spectrum. He described them as follows : 



(1.) " Blackened paper from which excess of nitrate of 

 silver has not been abstracted, under the influence of an 

 iodic salt. Produced by a November sun. N.B. View 

 it also transparently against the light." 



(2.) "Blackened paper under the influence of an iodic 



