1'MJS ATMOSPHERE 159 



CHAPTER V. 



Optical Phenomena. 



" Why do those cliffs of shadowy tint, appear 

 . More sweet than all the landscape smiling- near ? 

 'Tis distance lends enchantment to the view, 

 And robes the mountain in its azure hue." 



CampbdL 



IN the present chapter we shall describe and explain the general 

 optical appearance of the sky, and some of the more striking op- 

 tical phenomena connected with our present subject. When the 

 rays of the sun strike the minute particles of air, which, accord- 

 ing to circumstances, may be more or less dense, or charged with 

 watery vapor, they are either reflected, or transmitted ; in either 

 case some.times returning the most beautiful colors. It is a fact 

 to well known to need much illustration from us, that light, when- 

 ever it is refracted by any medium, such as glass or water, is al- 

 ways separated into the prismatic colors, whenever the surfaces 

 of the medium are curved, or inclined to each other. It is not 

 however, so generally understood, that these different colored 

 rays have different powers of penetrating through various media, 

 and that they move with different velocities. This however, is 

 susceptible of demonstration, and it is to this that the beautiful 

 colors of an autumnal sunset are owing. The red, violet and 

 orange rays have the greatest velocity, and penetrate the thick 

 dense strata of horizontal air, with the greatest facility, giving us 

 the rich and brilliant hues of sunset and sunrise, tinging the 

 morning and evening clouds with glowing red, and gold ; and 

 the sober twilight, with that purple fading into gray which is assum- 

 ed when the ruddy glare of sunset is tempered by the azure of 

 the sky. Since the red and yellow rays which compose white 

 light, are transmitted by the air, unattended by the blue rays, it 

 follows that these latter must be reflected, hence the beautiful 



