78 Intelligence and Miscellaneous Articles. 



the case of opaque bodies this property corresponds to what is 

 • more usually designated by " the proper colour of the body." 



I have just said that fluorescence is a general phenomenon of 

 transparent bodies. In fact, by operating on the purest liquids 

 (such, for example, as are obtained by condensing a gas like sul- 

 phurous acid, cyanogen, &c), it is proved that the illumination is 

 not extinguished when we look in a direction normal to the pencil 

 and to the plane of polarization of the incident light ; or if we view 

 natural light through a double prism, one of the two images of the 

 luminous ray never disappears completely. Nevertheless in study- 

 ing under better conditions some crystallized substances, I have 

 been able to recognize that quartz and rock-salt, when very pure, 

 exhibit not the slightest trace of fluorescence and are illuminated 

 very cleanly. To succeed in this experiment, it is necessary to po- 

 larize a large pencil of solar rays with a Eoucault normal-face 

 prism, and to concentrate it with a quartz lens cut parallel to the 

 axis, of 35-40 centims. focus. The principal sections of the polar- 

 izer and the lens are made to coincide ; and then their movements 

 are rendered conjoint. If the pencil thus concentrated pass through 

 the quartz either in the state of an ordinary or of an extraordinary 

 ray, a very distinct white trail is observed in the plane of polariza- 

 tion, which a Nicol completely extinguishes. On looking in a di- 

 rection normal to the plane of polarization, there is no illumina- 

 tion ; there remains not the least trace of fluorescence. When 

 the solar ray traverses the quartz along the optic axis, the disper- 

 sion of the plane of polarization has for its result the giving equal 

 illumination around the ray, and polarization is complete only in a 

 direction normal to the pencil. In this case chromatic illumination 

 should be observed, similar to that of sirup of sugar and all solu- 

 tions with rotatory power ; but with quartz it is not manifest ; 

 indeed we know that the mixed tints, much weakened, all affect a 

 uniform grey tint which the eye cannot distinguish. Yery pure 

 rock-salt is illuminated also like quartz, and is not fluorescent. 



It is not the same with. Iceland spar. All the specimens I have 

 examined are illuminated with orange-red more or less brilliant ; 

 but this coloured illumination is the same in the plane of polariza- 

 tion and normal to that plane ; it is not extinguished by a Mcol, 

 when the emergent rays, which necessarily undergo double refrac- 

 tion, remain superposed. This red light is due solely to fluores- 

 cence ; and the polarized illumination resembling that of quartz is 

 not appreciable. A peculiarity which I must notice is, that when 

 the incident solar thread is not polarized and traverses the spar 

 rhombohedron so as to give two pencils well separated, the fluores- 

 cence due to the ordinary ray appears more vivid and of a deeper 

 red than that of the extraordinary ray ; at least this is what I have 

 distinctly observed in two strongly fluorescent specimens. The 

 fluorescence of spar has besides been studied by M. Becquerel with 

 the phosphoroscope ; and I only confirm, by another method, his 

 observations. Colourless fluor-spar combines with more intensity 

 the two distinct properties of quartz and of spar : it gives a white 

 illumination, very bright in the plane of polarization, and a violet- 



