268 MEMORIAL OF JOSEPH HENRY. 



cence 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 ex- 

 tended these lists, including in them a large number of liquids. 

 He also subjected both the exciting rays (especially that of the elec- 

 tric spark), and the luminous emanation, to various treatment, by 

 reflection, refraction, polarization, etc. The Nicol prism was found 

 to obstruct this peculiar exciting 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 phosphoro- 

 genic 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 perceived by a rotation of the analyzer : " 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 sensitive substance : a fact re-discovered 

 by Professor George G. 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 spectro- 

 scope. 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 effect on the phosphorescent sub- 

 stance was found to be defined in location by the form of the open- 

 ing made in sheet-metal screens. Different portions of the electric 

 spark being tested by means of a narrow slit in the screen, the 

 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 



* That there should be such a difference between quartz and glass or mica, is cer- 

 tainly a remarkable circumstance. 



