On the Distribution of Chemical Force in the Spectrum. 423 



large body of evidence that might be adduced, I select a few 

 prominent instances. 



1st. Case of the Compounds of Silver. 



Silver is the basis of the most important photographic sensi- 

 tive substances. Its iodide, bromide, and chloride, darkening 

 with rapidity under the influence of the more refrangible rays, 

 have mainly been the cause of the misconception above alluded 

 to respecting the tripartite constitution of the spectrum. It is 

 necessary, therefore, to determine what are really the habitudes 

 of these substances. 



(1) If a spectrum be received on iodide of silver, formed on 

 the metallic tablet of the Daguerreotype, and carefully screened 

 from all access of extraneous light, both before and during the 

 exposure, on developing with mercury vapour an impression is 

 evolved in all the more refrangible regions. This stain corre- 

 sponds in character and position to the blackening effect which, 

 under like circumstances, would be found on ,^any common sen- 

 sitive silver paper. It is this which has given rise to the opinion 

 that the so-called actinic rays exist only in the upper part of 

 the spectrum. If, however, the action of the light be long con- 

 tinued, a white stain makes its appearance over all the less re- 

 frangible regions. It has a point of maximum, to which I shall 

 again presently refer. 



(2) But if the metallic tablet, during its exposure to the 

 spectrum, be also receiving diffused light of little intensity, as 

 the light of day or of a lamp, it will be found on developing 

 that the impression obtained differs strikingly from the pre- 

 ceding. Every ray that the prism can transmit, from below 

 the extreme red to beyond the extreme violet, has been active 

 The ultra-red heat-lines a, ^, y are present. It must be borne 

 in mind that the impression of these lines is a proof of proper 

 spectrum -action, and distinguishes it from that of diffused light 

 arising either from the atmosphere or from the imperfect trans- 

 parency of the prism — a valuable indication. The resulting 

 photograph shows two well-marked regions or phases of action. 

 On its general surface, which, having condensed the mercury 

 vapour, has the aspect of the high lights of the Daguerreotype, 

 and forms, as it were, the basis for the spectrum picture, there 

 is in the region of the more refrangible rays a bluish or olive- 

 coloured impression, the counterpart of the result described in 

 the foregoing paragraph. But in the region of the less refran- 

 gible rays no mercurial deposit has occurred, the place of those 

 rays being depicted in metallic silver, dark, and answering to 

 the shadows of the Daguerreotype. This protected portion, 

 which stands out in bold rehef from the white background, 



