SCIENTIFIC USE OF THE IMAGINATION. 113 



indubitable evidence to show that the light of our 

 firmament is scattered light. Proofs of the most co- 

 gent description could be here adduced; but we need 

 only consider that we receive light at the same time 

 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 rush 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 

 proportions 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 possi- 

 bility 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 transmitted 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 ether, but re- 

 flects them all alike. It exercises no selective fiction. 

 Now the cause of this may be that the cloud particles 



