THE SIX GATEWAYS OF KNOWLEDGE. 283 



quite dark, and the uranium glass glows with a 

 mysterious altered colour of a beautiful tint, re- 

 vealing the presence of invisible rays, by con- 

 verting them into rays of lower period, and so 

 rendering them visible to the eye. The discovery 

 of this property of uranium glass was made by 

 Professor Stokes, and the name of fluorescence, 

 from fluor spar, which he found to have the 

 same property, was given to it. It has since 

 been discovered that fluorescence and phos- 

 phorescence are continuous, being extremes of 

 the same phenomenon. I suppose most persons 

 here present know the luminous paint made from 

 sulphides of calcium and other materials, which, 

 after being steeped in light for a certain time, 

 keep on for hours giving out light in the darkness. 

 Persistence in emission of light after the removal 

 of the source, which is the characteristic of those 

 phosphorescent objects, is manifested also, as 

 Edmund Becquerel has proved, by the uranium 

 glass, and thus Stokes' discovery of fluorescence 

 comes to be continuous with the old known phe- 

 nomenon of phosphorescence, to which attention 



