346 POPULAR LECTURES AND ADDRESSES. 



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 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, oi 

 perhaps a fifty thousandth or a hundred thousandth 

 of a centimetre diameter, or perhaps little motes, 

 or lumps, or crystals of common salt, or particles 

 of dust, or germs of vegetable or animal species 

 wafted about in the air. Now what is the lumini- 



