. The evolution of forces . ce iiage 285. INVISIBLE PHOSPHORESCENCE 285 beforehand. By using a portrait camera with largeaperture, we obtain, by exposures varying from eightto fifteen days, images as perfect as those taken indaylight. The shadows vary at the will of theoperator, since they de-pend solely on the positiongiven to the statue duringinsolation (Figs. 24 and25). 3. Polarization. — Thedouble refraction and, con-sequently, polarization ofthis dark light are shownby the following experi-ment :—A strip of Icelandspar is introduced into theoptic system of the camerapreviously used, and t
. The evolution of forces . ce iiage 285. INVISIBLE PHOSPHORESCENCE 285 beforehand. By using a portrait camera with largeaperture, we obtain, by exposures varying from eightto fifteen days, images as perfect as those taken indaylight. The shadows vary at the will of theoperator, since they de-pend solely on the positiongiven to the statue duringinsolation (Figs. 24 and25). 3. Polarization. — Thedouble refraction and, con-sequently, polarization ofthis dark light are shownby the following experi-ment :—A strip of Icelandspar is introduced into theoptic system of the camerapreviously used, and thestatuette is replaced bytwo ojlass tubes in form ofa cross, filled with sulphideand fixed in a place settledbeforehand so as to obtaina good focus. By workingas before, a few days afterthe sulphide has ceased toemit light, we see, on oneof the axes of the cross,two partially superposedimages, of which the in-tensity is one half lessthan that of the part not duplicated—which is in conformity with the theory(Fig. 2G).. Fig. 26.—Polarization hy doublerefraction of the dark raysemitted hy bodies endowedwith invisible jthosphores-cence. Duplication of oneof the arms of the images are super-posed at the central part,and allow us to comparethe intensity of the dupli-cated parts (on the nega-tive tlie central part is, ofcourse, the darker). 286 THE EVOLUTION OF FORCES This experiment proves, at the same time, theemission of invisible radiations, their propagationin a straight hne, their refraction, and their aptitudefor polarization. 4. Composition of the rays emitted.—The perfectsharpness of the images obtained in the precedingexperiments already proves that the index ofrefraction of the lenses for dark rays is the sameas for visible light. Had it been otherwise, thepreliminary focussing by ordinary light would nothave been exact for rays of different wave-length,especially with the portrait camera employed, ofwhich the focal depth is almost nil. But that isonly
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