280 BULLETIN OF THE 



while this phosphorescence may be fully excited in the sensitive 

 body by rays which have passed through transparent sulphate 

 of lime, or through quartz, the effect is entirely arrested by a 

 plate of transparent mica, or glass. Henry by a long series of 

 experiments greatly extended these lists, including in them a 

 large number of liquids. He also subjected both the exciting 

 rays (especially of the electric spark) and the luminous emana- 

 tion, to various treatment, by reflection, refraction, polarization, 

 etc. The JS^icol prism was found to obstruct this peculiar ex- 

 citing ray so much as to permit scarcely any impression; but 

 what was remarkable and unexpected, a pile of thin mica plates 

 which seemed to cut off entirely the phusphorogeuic impression, 

 was found when placed obliquely at the best polarizing angle, to 

 distinctly excite a surviving luminous spot. On examination of 

 the phosphorescence excited by polarized light, no effect was 

 produced by a rotation of the analyser : " when the beam was 

 transmitted through crystals in different directions with reference 

 to their optical axis, no difference could be observed." The 

 phosphorescence was completely depolarized, as if taking an 

 entirely new origin in the sensiti\e substance: a fact re-dis- 

 covered by Professor Stokes some ten years later, with regard 

 to fluorescent emanations. 



That the phosphorogenic effect does not depend on a heating 

 of the substance, appeared to be shown by the fact that "the 

 lime becomes as luminous under a plate of alum as under a plate 

 of rock-salt." The emanation was examined by a prism of rock- 

 crystal, and by one of rock-salt : — science had not then the spec- 

 troscope. While the impression could be readily made by a 

 reflected beam from a metallic mirror, it failed entirely when 

 directed from a looking-glass. The luminous eflect on the phos- 

 phorescent substance was found to be defined in location by the 

 form of the opening made in sheet metal screens. On testing 

 with different portions of the electric spark by means of a narrow 

 slit in the screen, tlie two terminals of the spark were found to 

 be much more active (as measured by the subsequent duration 

 of the phosphorescence) than the middle portion. By a suitable 

 arrangement of double screens with three slits each, he was able 

 to make simultaneous star-like photographs on the substance, of 

 the two extreme portions of the spark and of a middle point: 

 and while the latter point " exhibited a feeble phosphorescence 

 for two or three seconds" only, the two former "continued to 

 glow for more than a minute:" and yet the middle of the spark 

 appeared to the eye quite as vivid as its extremities. It was 

 also observed that while a sensitive daguerreotype plate received 

 no impression from the electric spark, inversely another similar 

 plate exposed for several minutes to the direct light of the fuU 



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