

SU^^LiailT AXD COLOUES. 225 



the globe has been able to preserve its normal temperature, and has 

 become the theatre of life.* 



The atmosphere, which as the common vehicle of exchange is 

 ever in motion, is also the great agent by which nature receives 

 the wonderful colours that beautify her. It is owing to the re- 

 flection of the blue rays that the sky and the distant heights of 

 the horizon assume that beautiful azure hue, which varies with 

 the altitude of each region, the abundance of watery vapour, and the 

 contrast of the clouds. It is owing to the refraction undergone by 

 the luminous rays in passing obliquely through the aerial strata, 

 that the sun is announced every morning by the vague glimmers 

 of twilight, then by the splendours of dawn, and thus shows him- 

 self before the astronomical hour of his rising. It is also due to an 

 analogous phenomenon that in the evening he seems to slacken his 

 descent below the horizon^ and even after he has disappeared colours the 

 west for a long time with the purple of sunset. Without the gaseous 

 envelope of the earth we should never see those varied plays of light, 

 those changing harmonies of colour, those gradual transformations 

 of delicate shades, which form the marvellous beauty of our morn- 

 ings and evenings. The special works on meteorology describe at 

 leugth all these brilliant phenomena of the air, the rainbows, halos, 

 parhelions, and that splendid spectacle of the " after-glow " which 

 colours the snows and ice of the Alps with a rosy tint more than 

 twenty minutes after the sun has set. Nothing is so beautiful as this 

 phenomenon due to the contrast of the lower slopes which are already 

 in the shade, and the high peaks which the solar rays reflected above 

 the horizon still illuminate. When the Aiguille- Yerte is already 

 veiled in shadow, as well as the neighbouring summits of Mont Blanc, 

 the latter is truly transfigured by the light glittering on its snows. 

 " We might think we then saw a form foreign to the earth ; '' 

 then all at once the flame is extinguished, the colours so brilliant 

 vanish, " to give place to an aspect that we may truly call ca laverous, 

 for nothing approaches more nearly to the contrast between life and 

 death on the human face than this passing from the light of day to 

 the shadow of night on the high mountains." f 



The mirage is another singular optical efiect, due to the deviation 

 of the rays of light which traverse the atmosphere. When the 

 surface of the earth is much heated by the sun the lower strata of air 

 expand and often become lighter than the strata situated above. If 



* Tyndall, ffeat: 

 t Naker de Saussure, Annates de Chimie et de Fhtjsiqm, 1839. 



Q. 



