SCIENTIFIC USE OF THE IMAGINATION 121 



but is dispersed in space. Light thus reflected cannot, 

 therefore, be the light of the sky. 



But, though the sun's light is not reflected in this 

 fashion from the aerial layers to the earth, there is in- 

 dubitable evidence to show that the light of our firmament 

 is scattered light. Proofs of the most cogent 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 some- 

 thing 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 color 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 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. 

 SCIENCE YI 6 



