THE WAVE THEORY OF LIGHT 285 



due to spherules in the luminiferous ether, but little modi- 

 fied 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 condensation of vapour gives rise 

 to exactly the azure blue of the sky. 



Now the motion of the luminiferous ether relatively 

 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. 



The result in a general way is this: The light coming 

 from the blue sky is polarised in a plane through the 

 sun, but the blue light of the sky is complicated by a great 

 number of circumstances and one of them is this, that 

 the air is illuminated not only by the sun but by the 

 earth. If we could get the earth covered by a black cloth 

 then we could study the polarised light of the sky with 

 a simplicity which we cannot do now. There are, in 

 nature, reflections from the seas and rocks and hills and 

 waters in an infinitely complicated manner. 



Let observers observe the blue sky not only in winter 

 when the earth is covered with snow, but in summer when 

 it is covered with dark green foliage. This will help to 

 unravel the complicated phenomena in question. But the 

 azure blue of the sky is light produced by the reaction on the 

 vibrating ether of little spherules of water, of perhaps 

 a fifty thousandth or a hundred thousandth of a centi- 

 metre diameter, or perhaps little motes, or lumps, or crystals 

 of common salt, or particles of dust, or germs of vege- 

 table or animal species wafted about in the air. Now 

 what is the luminiferous ether? It is matter prodigiously 

 less dense than air millions and millions and millions of 



