SECT. XXII. FLUORESCENCE. 197 



tions from the smaller number of molecules, which thus became 

 really new centres of light, different from the sun's light, though 

 owing to it ; the one celestial, the other terrestrial ; and the latter 

 vibrations being more rapid than those of the blue light, their re- 

 f rangibility was less, and therefore their colour lower in the pris- 

 matic scale. Mr. Power computed from his formula?, that fluor- 

 escent light is produced by undulations which are a major or 

 minor third below the pitch of the general vibration of the 

 medium that is to say, below the vibrations which the whole 

 molecules of the body most readily assume. 



Professor Stokes, of Cambridge, who made the preceding ex- 

 periment, found that the chemical rays from a point in the 

 solar spectrum produced, in a solution of the sulphate of qui- 

 nine, light of a sky-blue colour, which emanates in all directions 

 from the liquid, and that this blue fluorescent light contains, 

 when analysed, all the rays of the spectrum ; hence he inferred 

 that the dispersive power or fluorescence had lowered the re- 

 frangibility of the chemical rays, so as to make them visible : 

 and Sir David Brewster observes that the new spectrum, of all 

 colours into which they were transformed, must possess the 

 extraordinary property of being a luminous spectrum, either 

 without chemical rays or full of them. The dispersion in the 

 quinine solution is greatest near the surface, but the blue ema- 

 nation proceeds from every part of the liquid ; and Sir John 

 Herschel, who discovered the fluorescent property in this liquid, 

 and gave it the name of epipolic light, found that the remainder 

 of the beam, when it issued from the solution, though not 

 apparently different from the incident white light, is yet so 

 much changed in passing through the liquid, that it is no longer 

 capable of producing fluorescence, though still capable of common 

 dispersion. The blue light from the solution of quinine, when 

 examined, consisted of rays extending over a great part of the 

 spectrum. 



By passing a sunbeam through a bluish kind of fluor-spar, 

 Sir David Brewster perceived that the blue colour is not super- 

 ficial, as it appears to be, but that some veins in the interior of 

 the crystal disperse blue light, others pink, and even white 

 light ; in short, he met with fluorescence in such a variety of 

 substances, that he concludes it may prevail more or less in the 

 greater number of solids and liquids. 



