196 FLUORESCENCE. SECT. XXIf. 



a great number of organic substances, have the property of 

 diminishing the refrangibility of light by internal dispersion, 

 consequently of increasing the length of the waves, and lowering 

 the colour in the prismatic scale ; it is therefore called degraded 

 light, or fluorescence, because first discovered in fluor-spar. 



If a piece of glass coloured by cobalt be fixed in a hole in a 

 window-shutter of a dark room, a slab of white porcelain placed 

 near it will appear blue ; but if the slab be viewed through a 

 yellow glass coloured by silver, it will appear to be almost quite 

 black, because the yellow glass absorbs all the rays transmitted 

 by the blue glass. If, however, a piece of canary-glass be laid 

 on the slab while it is dark, every part of the canary-glass will 

 shine as if it were self-luminous, and with so bright a light that 

 anything written on the slab that was invisible before may now 

 be distinctly read. Such is the singular phenomenon of internal 

 dispersion, degraded light, or fluorescence. The brightness is 

 by no means due to phosphorescence, because the canary-glass 

 only shines when under the influence of the active or blue rays, 

 whereas phosphorescent bodies shine by their own light the 

 latter has independent, the former dependent, emission ; it is 

 possible, however, that a connexion may hereafter be traced be- 

 tween them. 



It appears from the analytical investigation of this pheno- 

 menon that the vibrations of the fluorescent substance are ana- 

 logous to those of a sonorous body, as a bell or musical cord, 

 which give the fundamental note and its harmonics. Now since 

 there is a reciprocal action between the molecules of matter and 

 light, when the light of the sun is absorbed by a substance 

 capable of fluorescence, it puts the whole of its molecules into 

 vibrations the same as its own, analogous to the fundamental 

 note, while at the same time a certain number of molecules take 

 more rapid vibrations exactly like the harmonics. The latter 

 form new centres of light throughout the substance, which 

 impart their vibrations to the ethereal medium around, and con- 

 stitute fluorescence or degraded light. For example, in the 

 experiment that has been described, the blue light imparted its 

 own vibrations to all the molecules of the canary-glass, and also 

 more rapid vibrations to a certain number of them. All of the 

 blue rays were excluded by the yellow glass held before the eye ; 

 but it was pervious to the rays emanating in more rapid vibra- 



