. The evolution of forces . er by calcination or, without any rise in thetemperature, by exposing the body to dark radiationsof a certain Avave-lensfth. § 5.—Invisible Phosphorescence preceding theVisible The invisible phosphorescence which, as we havejust seen, follows the visible, may precede it. Thisis proved by the following experiments. Let us take a screen made of a phosphorescentbody which is slowly impressed by light, such assulphide of strontium, and deprive it, by heat, of allresidual phosphorescence. Let us then place it inthe plate-carrier of a camera halving a diaphragmtimed to it
. The evolution of forces . er by calcination or, without any rise in thetemperature, by exposing the body to dark radiationsof a certain Avave-lensfth. § 5.—Invisible Phosphorescence preceding theVisible The invisible phosphorescence which, as we havejust seen, follows the visible, may precede it. Thisis proved by the following experiments. Let us take a screen made of a phosphorescentbody which is slowly impressed by light, such assulphide of strontium, and deprive it, by heat, of allresidual phosphorescence. Let us then place it inthe plate-carrier of a camera halving a diaphragmtimed to ith of a second, point the apparatus towardsthe sky, and uncover the diaphragm so that the plateis exposed to light for that period. On openingthe frame afterwards in the dark, we note that thesulphide is not luminous, but it will suttice to placeit on a plate heated to 200° C for illumination tooccur. Its short exposure to the light had thusgiven it an invisible phosphorescence. The experiment may be effected more simply. Fig. 28. Photographic reproduction in the dark of statuettes hy dark raysof great wave-length. They make the objects luminous by combining with other darkrays emitted by the statuettes themselves. To face page 292. INVISIBLE PHOSPHORESCENCE 293 with other bodies—Iceland spar, for example. Thiscompound acquires a very slight visible phospho-rescence by heat and none by light, but if in-solated, and then heated to 200° C, it shines withbrilliance for some minutes, v/hich proves that thelight communicates to it a certain quantity of in-visible phosphorescence. This series of operationscan be repeated indefinitely—that is to say, we canrestore to the spar the same luminosity by heat afterhavino- insolated it. § G.—Comparative Effects on FhospJtorescence of theInfra-red Radiations and of Heat The first observers, having studied the action ofthe various parts of the spectrum on bodies capableof phosphorescence, very soon noted that the radia-tions going f
Size: 1354px × 1845px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, bookid, booksubjectradioactivity
