. The evolution of forces . activity during theexposure of the photographic plate to the light;(2) the blue rays which chiefly act on the photo-graphic plate are by no means the most active agentsin the dissociation of matter. The elucidation of this last point led me to inquirewhich were the radiations with the greatest actionon the photographic impression. In order to make these evident I exposed a photo-graphic plate behind a spectroscope, and watched inwhich region the impression commenced. It alwaysbegan in the blue, and never in the violet or theultra-violet. TRANSFORMATION OF MATTER INT
. The evolution of forces . activity during theexposure of the photographic plate to the light;(2) the blue rays which chiefly act on the photo-graphic plate are by no means the most active agentsin the dissociation of matter. The elucidation of this last point led me to inquirewhich were the radiations with the greatest actionon the photographic impression. In order to make these evident I exposed a photo-graphic plate behind a spectroscope, and watched inwhich region the impression commenced. It alwaysbegan in the blue, and never in the violet or theultra-violet. TRANSFORMATION OF MATTER INTO LIGHT 213 Although the Jena-glass prisms of my spectroscopesallow nearly all the solar ultra-violet to pass, they,however, absorb a portion, and it might be objectedto the above experiments that the feeble action ofthe violet and the ultra-violet was the result of thisabsorption. I therefore asked M. do Wattcville, whoowns a large spectroscope with a quartz prism, verytransparent to ultra-violet rays, to repeat my experi-. Fig. IG.—Photographs of Solar Spectra to shoio the rariatioib of therisible rays used durinij exposure. The lowest spectrum wasobtained with half a seconds exposure. There is no impressionexcept in the blue. Jhe blue rays are therefore the only onesmade use of in instantaneous photography. The two spectraplaced next above were obtained with successivelj longerexposures. The upper spectrum extends some distance intothe ultra-violet. By still further prolonging the exposure Ihave obtained with ordinary plates impressions even up tothe red ; but the rest of the photograph is then useless. ments with his instrument. They gave resultsidentical to those set forth above. The impressionalways begins in the blue, and is only propagatedsome time afterwards into the ultra-violet. It maybe gathered from this that the use of objectives ofquartz or of glass very transparent to the ultra-violet would offer absolutely no advantage in in-stantaneous photography. 214 THE E
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