284 KELVIN 



sky which is illustrated by this spherule imbedded in an 

 elastic solid (FiG. 121). I want to explain to you in 

 two minutes the mode of vibration. Take the simplest 

 plane-polarised light. Here is a spherule which is pro- 

 ducing it in an elastic solid. Imagine the solid to extend 

 miles horizontally and miles up and down, and imagine 

 this spherule to vibrate up and down. It is quite clear 

 that it will make transverse vibrations similarly in all 

 horizontal directions. The plane of polarisation is defined 

 as a plane perpendicular to the line of vibration. Thus, 

 light produced by a molecule vibrating up and down, 

 as this red globe in the jelly before you, is polarised in a 

 horizontal plane because the vibrations are vertical. 



Here is another mode of vibration. Let me twist this 

 spherule in the jelly as I am now doing, and that will 



FIG. 121 Vibrating Spherule Imbedded in an Elastic Solid 



produce vibrations, also spreading out equally in all horizon- 

 tal directions. When I twist this globe round it draws 

 the jelly round with it; twist it rapidly back and the 

 jelly flies back. By the inertia of the jelly the vibrations 

 spread in all directions and the lines of vibration are 

 horizontal all through the jelly. Everywhere, miles away 

 that solid is placed in vibration. You do not see the 

 vibrations, but you must understand that they are there. 

 If it flies back it makes vibration, and we have waves 

 of horizontal vibrations travelling out in all directions 

 from the exciting molecule. 



I am now causing the red globe to vibrate to and fro 

 horizontally. That will cause vibrations to be produced 

 which will be parallel to the line of motion at all places 

 of the plane perpendicular to the range of the exciting 

 molecule. What makes the blue sky? These are exactly 

 the motions that make the blue light of the sky, which is 



