210 POPULAR LECTURES AND ADDRESSES. 



phosphorescence ; but he found some seeming 

 contrasts between the two, which prevented him 

 from concluding fluorescence to be in reality a 

 case of phosphorescence. 



In the course of a comparison between the two 

 phenomena (sections 221 to 225 of his 1852 paper), 

 the following statement is given : " But by far 

 the most striking point of contrast between the 

 two phenomena consists in the apparently in- 

 stantaneous commencement and cessation of the 

 illumination, in the case of internal dispersion 

 when the active light is admitted and cut off. 

 There is nothing to create the least suspicion of 

 any appreciable duration in the effect. When 

 internal dispersion is exhibited by means of an 

 electric spark, it appears no less momentary than 

 the illumination of the landscape by a flash of 

 lightning. I have not attempted to determine 

 whether any appreciable duration could be made 

 out by means of a revolving mirror." The in- 

 vestigation here suggested has been actually made 

 by Edmund Becquerel, and the question Is there 

 any appreciable duration in the glow of fluor- 



