210 Sir William Tliomson [Feb. 2, 



its periods being all less than those of the exciting ligbt, tbat led 

 Stokes to distinguish this illumination, which you see in the uranium 

 glass,* from the mere molecular illumination (always polarised 

 partially if not completely, and always of the same period as that of 

 the exciting light) which we were looking at previously in Dr. 

 Tyntlall's experiment. 



Stokes gave the name of fluorescence to the glowing with light of 

 larger period than the exciting 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 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 (sec- 

 tions 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 instantaneous commence- 

 ment and cessation of the illumination, in the case of internal 

 dispersion when the active light is admitted and cut ofi^. 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 

 a 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 investigation here suggested has been 

 actually made by Edmund Becquerel, and the question — Is there any 

 appreciable duration in the glow of fluorescence ? — has been answered 

 aiflrmatively by this beautiful and simple little machine before you, 

 which he invented for the purpose. The experiment giving the 

 answer is most interesting, and I am sure you will see it with 

 pleasure. It consists of a flat circular box, with two holes facing one 

 another in the flat sides near the circumference ; inside are two disks, 

 carried by a rapidly revolving shaft, by which the holes are alter- 



* The same phenomenon is to be seen splendidly in sulphate of quinine. An 

 interesting experiment may be made by writing on 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 phenomenon 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 experimental 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). 



