276 



NATURE 



\July 19, 183; 



prism in your hand, you would learn that when you see 

 light at all, its plane of polarisation is in the plane through 

 your eye and the axis of the tube ; and I hope you all 

 now perfectly understand the proof that the direction of 

 vibration is perpendicular to this plane. 



Now I want to bring before you something which 

 was taught me a long time ago by Professor Stokes ; 

 and year after year I have begged him to publish it, 

 but he has not done so, and so I have asked 

 him to allow me to speak of it tonight. It is a 

 dynamical explanation of that wonderful phenomenon 

 called fluorescence or phosphorescence. The principle is 

 mechanically represented by this model (described above 



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Fl(5. 10. — Diagram showing the different amplitudes of vibration of a row of 

 particles oscillating in a period less than their least wave-period. 



with reference to Fig. 2). A simple harmonic motion 

 is, as you now see, sustained by my hand in the uppermost 

 bar, in a period of about four seconds. You see that a 

 regular wave-motion travels down the line of molecules 

 represented by these circular disks on the ends of the 

 bars ; and the energy continually given to the top bar, by 

 my hand, is continually consumed in heating the basin of 

 treacle and water at the foot. I now remove my hand and 

 leave the whole system to itself. The very considerable 

 sum of kinetic and potential energies of the large masses 

 and spiral springs, attached to the top bar, is gradually 

 spent in sending the diminishing series of waves down the 



line, and is ultimately converted into heat in the treacle 

 and water. You see that about half of the amplitude of 

 vibration, and therefore three-fourths of the energy is 

 lost in half a minute. 



You will see on quickening the oscillation how very 

 different the result will be. The quick oscillations which 

 I now give to the top bar (the period having been reduced 

 to about one and a half seconds), is incapable of sending 

 waves along the line of molecules ; and it is that rapid 

 oscillation of the particles which, according to Stokes, con- 

 stitutes latent or stored-up light. Remark now that when 

 I remove my hand from the top bar, as no waves travel 

 down the line, no energy is spent in the treacle ; and the 

 vibration goes on for ever (or, to be more exact, say for 

 one minute) as you see, with no loss (or, to be quite in 

 accordance with what we see, let me say scarcely any 

 sensible loss). This is a mechanical model correctly illus- 

 trating the dynamical principle of Stokes' explanation 

 of phosphorescence or stored-up light, stored as in the now 

 well-known luminous paint ; of which you see the action 

 in this specimen, and in the phosphorescent sulphides of 

 lime in these glass tubes kindly lent by Mr. De La Rue. 

 (Experiments shown.) 



Now I will show you Stokes' phenomenon of fluor- 

 escence in a piece of uranium glass. I hold it in the 

 beam from the electric lamp dispersed by the prism as 

 you see. You see the uranium glass now visible by being 

 illuminated by invisible rays. The rays by which it is 

 illuminated even before it comes into the visible rays are 

 manifestly invisible so far as the screen receiving the 

 spectrum is a test of visibility ; because the uranium glass, 

 and my hands holding it, throw no shadow on the screen. 

 Also you see the uranium glass which I hold in my hand 

 in the ultra-violet light, while you do not see my hand. I 

 now bring it nearer the place where you see the ail (or 

 rather the dust in it) illuminated by the violet light : still 

 no shadow on the screen, but the uranium glass in my 

 hand glowing more brilliantly with its green light of very 

 mixed constitution, consisting of waves of longer periods 

 than that of the ultra-violet, which the incident light, of 

 shorter period than that of violet light, causes the particles 

 of the uranium glass to emit. This light is altogether un- 

 polarised. It was the absolute want of polarisation, and 

 the fact of its periods being all less than those of the 

 exciting light, that led Stokes to distinguish this illumina- 

 tion, 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. 

 Tyndall'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 



1 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 pre- 

 sented by sulphate of quinine and many other vegetable solutions, and si me 

 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 "epipoltc dispersion" {Phii. Trans., 1845). A complete ex- 

 perimental analysis of the phenomem n, 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" (Thil. Trans., May 27, 

 1852). 



