Scientific memoirs : being experimental contributions to a knowledge of radiant energy / by John William Draper.

  • John William Draper
Date:
1878
Catalogue details

Licence: Public Domain Mark

Credit: Scientific memoirs : being experimental contributions to a knowledge of radiant energy / by John William Draper. Source: Wellcome Collection.

    410/488 (page 406)
    darkening with rapidity under tlie influence of the more refrangible rays, have mainly been the cause of the mis- conception above alluded to respecting the trij)artite con- stitution of the sj)ectrum. 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 va]3or an impression is evolved in all the more refrangible regions. This stain corresjDouds in character and j)osition 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, ^vhich, having con-