60 FLUORESCENCE. PARTI. 



tions to the surrounding ether. Those having the same 

 velocity with the chemical rajs make no sensible im- 

 yicvuA/c pression on our eyes ; but the moro rapid vibrations 

 come within the limits of the visible spectrum; they 

 have consequently a lower refrangibility, and shine as 

 visible light. It is called degraded light on account of 

 its lower position in the prismatic scale, but more fre- 

 quently fluorescent light, because fluor spar was the 

 first solid known to possess the property. A number of 

 substances are fluorescent, both solid and liquid, organic 

 and inorganic. 



If in a dark room a non-fluorescent body be illumi- 

 nated by a sunbeam passing through glass stained deep 

 blue by cobalt, it will reflect blue light; but it will 

 appear to be perfectly black if it be viewed through 

 glass tinged yellow by silver ; while a piece of canary 

 glass, which is highly fluorescent, will shine with a 

 vivid light under the same circumstances. All the 

 molecules of the canary glass give back to the ether 

 the undulations that have been impressed on them by 

 the blue light ; while a certain number of them possess 

 . I the power of receiving and giving back more^ rapid 

 \ vibrations to the ether. The yellow glass held before 

 the eye is impervious to the undulations of the blue 

 rays, but transmits those of the fluorescent light, which 

 emanate from the smaller number of molecules, and 

 which thus become in reality new centres of light, 

 different from the sun's light, though dependent upon 

 it : the one terrestrial, the other celestial. Since the 

 vibrations of the fluorescent light are more rapid than 

 those of the blue light their colour is lower, in the 

 prismatic scale. The vibrations of the molecules in a 

 fluorescent substance are analogous to those of a musical 

 cord, which give the fundamental note or pitch and its 

 harmonics, for the whole of the musical cord while 

 vibrating the fundamental note divides itself sponta- 



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