406 CHEMICAL FORCE IN THE SPECTKUM. [MEMOIR XXIX. 



darkening with rapidity under the influence of the more 

 refrangible rays, have mainly been the cause of the mis- 

 conception above alluded to respecting the tripartite con- 

 stitution of the spectrum. It is necessary, therefore, to 

 determine what are really the habitudes of these sub- 

 stances. 



(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 vapor an impression is evolved in all the 

 more refrangible regions. This stain corresponds in 

 character and position to the blackening effect which 

 under like circumstances would be found on any com- 

 mon sensitive 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 continued, a white stain 

 makes its appearance over all the less refrangible re- 

 gions. 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 in- 

 tensity, as the light of day or of a lamp, it will be found 

 on developing that the impression obtained differs strik- 

 ingly from the preceding. Every ray that the prism can 

 transmit, from below the extreme red to beyond the ex- 

 treme violet, has been active. The ultra-red lines a, /3, y 

 are present. It must be borne in mind that the impres- 

 sion 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 result- 

 ing photograph shows two well-marked regions or phases 

 of action. On its general surface, which, having con- 



