THE SKY. 139 



action of the light, the azure becomes less deep ; while 

 later on a milkiness, such as we often observe in nature, 

 takes the place of the purer blue. Finally the particles 

 become large enough to reflect all the light- waves, and 

 then the suspended ' actinic cloud ' diffuses white light. 

 It must occur to the reader that even in the absence 

 of definite clouds there are considerable variations in 

 the hue of the firmament. Everybody knows, moreover, 

 that as the sky bends towards the horizon, the purer 

 blue is impaired. To measure the intensity of the colour 

 De Saussure invented a cyanometer, and Humboldt has 

 given us a mathematical formula to express the diminu- 

 tion of the blue, in arcs drawn east and west from the 

 zenith downwards. This diminution is a natural con* 

 sequence of the predominance of coarser particles in the 

 lower regions of the atmosphere. Were the particles 

 which produce the purer celestial vault all swept away, 

 we should, unless helped by what has been called 

 4 cosmic dust,' look into the blackness of celestial space. 

 . And were the whole atmosphere abolished along with its 

 suspended matter, we should have the * blackness ' 

 spangled with steady stars; for the twinkling of the 

 stars is caused by our atmosphere. Now, the higher we 

 ascend, the more do we leave behind us the particles 

 which scatter the light; the nearer, in fact, do we ap- 

 proach to that vision of celestial space mentioned a 

 moment ago. Viewed, therefore, from the loftiest 

 Alpine summits, the firmamental blue is darker than it 

 is ever observed to be from the plains. 



It is thus shown that by the scattering action of 

 minute particles the blue of the sky can be produced ; 

 but there is yet more to be said upon the subject. Let 

 the natural sky be looked at on a fine day through a 

 piece of transparent Iceland spar cut into the form 

 known as a Nicol prism. It may be well to begin by 



