Chemical Force in the Spectrum, 433 



traneous and simultaneously acting light is checked^ and the 

 effect of previously acting 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 (February 1843) Herschel 

 gave a detailed description of these spectrum-impressions. He 

 was disposed to refer the appearance they present to the phe- 

 nomena of their films, but at the same time pointed out the 

 difficulties in the way of that explanation. 



He also sent me three proofs he had obtained on ordinary 

 sensitive paper, darkened by exposure to light, then washed 

 with a solution of iodide of potassium, and placed in the spec- 

 trum. 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 transpa- 

 rently against the light.^^ 



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

 when no excess of nitrate of silver exists in the paper. ^^ 



(3) '^^ Action of spectrum under iodic influence when very 

 little nitrate of silver remains in excess in the paper. To be 

 viewed also transparently.^^ 



These paper photographs I still preserve. They are as per- 

 fect as when first made. The diff'erent coloured spaces of the 

 spectrum are marked upon them with pencil. The appearances 

 they respectively present are as follows : — 



(1) is bleached by the more refrangible rays, and blackened 

 deeply from the yellow to the ultra-red. 



(2) is bleached from the ultra-invisible red to the ultra-violet. 

 A maximum occurs abruptly about the blue. 



(3) has the same upper spectrum as the others, a bleached 

 dot in the centre of the yellow, and a darkened space in the 

 extreme red. The action has reached from the ultra-red to the 

 ultra-violet. 



In HerscheFs opinion, these eff'ects in the less refrangible 

 region are connected with the drying of the paper. It is well 

 known that paper in a damp condition is more sensitive than 

 such as is dry. 



But obviously this condition does not obtain in the case of 

 the Daguerreotype operation, which is essentially a dry process. 



In 1846 MM. Foucault and Fizeau, having repeated the ex- 

 periment originally made by me, presented a communication to 

 the French Academy of Sciences, to the efi'ect that when a silver 

 tablet which has been sensitized by exposure to iodine and bro- 

 mine, and then impressed by light, is exposed to the spectrum, 



Phil. Mag. S. 4. Vol. 44. No. 295. Dec. 1872. 2 F 



