THE SIZE OF ATOMS. 203 



beam be of plane polarised light that is to say, 

 light of which all the vibrations are parallel to 

 one line and if you look at the tube in the 

 direction perpendicular to this line and to the 

 length of the tube, you will see light of which 

 the vibrations will be parallel to that same line. 

 But if you look at the tube in any direction 

 parallel to this line, you will see no light ; and 

 the line along which you see no light is the 

 direction of the vibrations in the exciting beam ; 

 and this direction, as we now see, is the direction 

 perpendicular to what is technically called the 

 plane of polarisation of the light. Here, then, 

 you have Stokes' experimentum crucis by which 

 ' he has answered, as seems to me beyond all doubt, 

 the old vexed question Whether is the vibration 

 perpendicular to, or in the plane of polarisation ? 

 To show you this experiment, instead of using 

 unpolarised light for the exciting beam, as in 

 the previous experiment, and holding a small 

 Nicol's prism in my hand and telling you what 

 I saw when I looked through it, I place, as is 

 now done, this great Nicol's prism in the course 



