THE SKY.. 139 



continued action of the light, the azure becomes less 

 deep; while later on a milkiness, such as we often ob- 

 serve 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, more- 

 over, 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 ex- 

 press the diminution of the blue, in arcs drawn east and 

 west from the zenith downwards. This diminution is 

 a natural consequence 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 ' cosmic dust,' look into the blackness of celestial 

 space. And were the whole atmosphere abolished along 

 with its suspended matter, we should have the ' black- 

 ness ' 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 mo- 

 ment 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 



