THE SIZE OF ATOMS. 209 



illumination (always polarised partially if not com- 

 pletely, and always of the same period as that 

 of the exciting light) which we were looking at 

 previously in Dr. Tyndall's experiment. 



Stokes gave the name of fluorescence to the 

 glowing with light of larger period than the excit- 

 ing light, because it is observed in fluor spar, and 

 he wished to avoid all hypothesis in his choice of 

 a name. He pointed out a strong resemblance 

 between it and the old known phenomenon of 



a white paper screen, with a finger or a brush dipped in a solution 

 of sulphate of quinine. The marking is quite imperceptible in 

 ordinary light ; but if a prismatic spectrum be thrown on the screen, 

 with the ultra-violet invisible light on the part which had been 

 written on with the sulphate of quinine, the writing is seen glowing 

 brilliantly with a bluish light, and darkness all round. The pheno- 

 menon presented by sulphate of quinine and many other vegetable 

 solutions, and some minerals, as, for instance, fluor spar, and various 

 ornamental glasses, as a yellow Bohemian glass, called in commerce 

 "canary glass" (giving a dispersed greenish light), had been 

 discovered by Sir David Brewster ( Transactions, Royal Society of 

 Edinburgh, 1833, and British Association, Newcastle, 1838), and 

 had been investigated also by Sir John Herschel, and by him called 

 " epipolic dispersion" (Phil. Trans. 1845). A complete experi- 

 mental analysis of the phenomenon, showing precisely what it was 

 that the previous observers had seen, and explaining many singularly 

 mysterious things which they had noticed, was made by Stokes, 

 and described in his paper, "On the Change of Refrangibility of 

 Light" (Phil. Trans. May 27, 1852). 



