66 



LECTURES AND ESS A YS 



waves of red light are split into exact 

 halves, then, to preserve the light white, 

 the waves of yellow, orange, green, and 

 blue must also be split into exact halves. 

 In short, the reduction must take place, 

 not by absolutely equal quantities, but 

 by equal fractional parts. In white light 

 the preponderance, as regards energy, of 

 the larger over the smaller waves must 

 always be immense. Were the case 

 otherwise, the visual correlative, blue, of 

 the smaller waves would have the upper 

 hand in our sensations. 



Not only are the waves of ether re- 

 flected by clouds, by solids, and by 

 liquids, but when they pass from light 

 air to dense, or from dense air to light, 

 a portion of the wave-motion is always 

 reflected. Now, our atmosphere changes 

 continually in density from top to bottom. 

 It will help our conception if we regard 

 it as made up of a series of thin con- 

 centric layers, or shells of air, each shell 

 being of the same density throughout, a 

 small and sudden change of density 

 occurring in passing from shell to shell. 

 Light would be reflected at the limiting 

 surfaces of all these shells, and their 

 action would be practically the same as 

 that of the real atmosphere. And now 

 I would ask your imagination to picture 

 this act of reflection . What must become 

 of the reflected light ? The atmospheric 

 layers turn their convex surfaces towards 

 the sun ; they are so many convex 

 mirrors of feeble power ; and you will 

 immediately perceive that the light regu- 

 larly reflected from these surfaces cannot 

 reach the earth at all, but is dispersed in 

 space. Light thus reflected cannot, there- 

 fore, 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 indubitable 

 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 con- 

 sider 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 sus- 

 pended 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 account- 

 ing 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 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 imagina- 

 tion we may hope to penetrate this 

 mystery. 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 comparison with the waves of 

 ether, 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 surfaces the existing differences 

 of magnitude among the waves of ether 

 may disappear. But supposing the re- 

 flecting particles, instead of being very 

 large, to be very small in comparison 

 with the size of the waves. In this case, 

 instead of the whole wave being fronted 

 and thrown back, a small portion only is 



