426 VRA GMENTS OF SCIENCE. 



become of the reflected light? The atmospheric layers 

 turn their convex surfaces toward the sun; they are so 

 many convex mirrors of feeble power; and you will 

 immediately perceive that the light regularly reflected from 

 these surfaces cannot reach the earth at all, 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 fash- 

 ion from the aerial layers to the earth, there is indubitable 

 evidence to show that the light of our firmament is scat- 

 tered 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 direc- 

 tion 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 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 sun- 

 rise 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 conld 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. 



By the scientific use of the imagination we may hope to 

 penetrate this rnysteiy. The cloud takes no note of size 

 on the part of the waves of ether, 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 com- 

 parison with the waves of ether, as to reflect them all 

 indifferently. A broad cliff reflects an Atlantic roller as 



