SCIENTIFIC USE OF THE IMAGINATION. 113 



from all parts of the hemisphere of heaven. The light 

 of the firmament comes to us across the direction of 

 the solar rays, and even against the direction of the 

 solar rays ; and this lateral and opposing rash of wave- 

 motion can only be due to the rebound of the waves 

 from the air itself, or from something suspended in the 

 air. It is also evident that, unlike the action of clouds, 

 the solar light is not reflected by the sky in the pro- 

 portions which produce white. The sky is blue, which 

 indicates an excess of the shorter waves. In accounting 

 for the colour of the sky, the first question suggested 

 by analogy would undoubtedly be, Is not the air blue ? 

 The blueness of the air has, in fact, been given as a 

 solution of the blueness of the sky. But how, if the 

 air be blue, can the light of sunrise and sunset, which 

 travels through vast distances of air, be yellow, orange, 

 or even red ? The passage of white solar light through 

 a blue medium could by no possibility redden the light. 

 The hypothesis of a blue air is therefore untenable. 

 In fact the agent, whatever it is, which sends us the 

 light of the sky, exercises in so doing a dichroitic 

 action. The light reflected is blue, the light trans- 

 mitted is orange or red. A marked distinction is thus 

 exhibited between the matter of the sky, and that of 

 an ordinary cloud, which exercises no such dichroitic 

 action. 



By the scientific use of the imagination we may 

 hope to penetrate this mystery. The cloud takes no 

 note of size on the part of the waves of aether, but reflects 

 them all alike. It exercises no selective action. Now 

 the cause of this may be that the cloud particles are 

 so large, in comparison with the waves of aether, as to 

 reflect them all indifferently. A broad cliff reflects an 

 Atlantic roller as easily as a ripple produced by a sea- 

 bird's wing ; and in the presence of large reflecting sur- 



YOL. II. I 



