204 POPULAR LECTURES AND ADDRESSES. 



of the beam of light before it enters the tube. I 

 now turn the Nicol's prism into different directions 

 and turn the apparatus round, so that, sitting in 

 all parts of the theatre, you may all see the tube 

 in the proper direction for the successive pheno- 

 mena of "light," and "no light." You see them 

 now exactly fulfilling the description which I 

 gave you in anticipation. If each of you had 

 a Nicol's prism in your hand, you would learn 

 that when you see light at all, its plane of pola- 

 risation 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 Pro- 

 fessor Stokes : 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 to-night. 

 It is a dynamical explanation of that wonderful 

 phenomenon called fluorescence or phosphorescence. 

 The principle is mechanically represented by this 

 model (described above with reference to Fig. 34, 



