THE WAVE THEORY OF LIGHT. 



343 



a simple observation which you all can make 

 for yourselves. 



Lastly, I must tell you about the colour of the 

 blue sky which is illustrated by this spherule 

 imbedded in an elastic solid (Fig. 49). I want to 

 explain to you in two minutes the mode of vibra- 

 tion. Take the simplest plane-polarised light. 

 Here is a spherule which is producing it in an 



\\\ 



FIG. 49. Vibrating Spherule Imbedded in an Elastic Solid. 



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 vibra- 

 tions similarly in all horizontal directions. The 

 plane of polarisation is defined as a plane perpen- 

 dicular to the line -of vibration. Thus, light pro- 

 duced by a molecule vibrating up and down, as this 

 red globe in the jelly before you, is polarised in 



