THE WAVE THEORY OF LIGHT. 345 



motions that make the blue light of the sky, 

 which is due to spherules in the luminiferous 

 ether, but little modified by the air. Think of 

 the sun near the horizon, think of the light of the 

 sun streaming through and giving you the azure 

 blue and violet overhead. Think first of any one 

 particle and think of it moving in such a way 

 as to give horizontal and vertical vibrations and 

 circular and elliptic vibrations. 



You see the blue sky in high pressure steam 

 blown into the air ; you see it in the experiment 

 of Tyndall's blue sky in which a delicate condensa- 

 tion of vapour gives rise to exactly the azure blue 

 of the sky. 



Now the motion of the luminiferous ether re- 

 latively to the spherule gives rise to the same 

 effect as would an opposite motion impressed 

 upon the spherule quite independently by an 

 independent force. So you may think of the 

 blue colour coming from the sky as being produced 

 by to and fro vibrations of matter in the air, which 

 vibrates much as this little globe vibrates imbedded 

 in the jelly. 



