12S TRANSMISSION OF LIGHT. 



Our atmosphere, charged with aqueous vapour, serves 

 to shield us from the intense action of the solar powers. 

 By it we are protected from the destructive influences 

 of the sun's light and heat; enjoy those modified 

 conditions which are most conducive to the healthful 

 being of organic forms ; to it we owe " the blue sky 

 bending over all," and those beauties of morning and 

 evening twilight of which 



Sound and motion own the potent sway, 



Responding to the charm with its own mystery. 



To defective transparency, or rather to the different 

 degrees of it, we must attribute, in part, the colours of 

 permeable media. Thus, a glass or fluid appears yellow 

 to the eye, because it has the property of admitting the 

 permeation of a larger quantity of the yellow rays than 

 of any others ; red, because the red rays pass it with 

 the greatest freedom ; and so on for every other colour. 

 In most cases the powers of transmission and of reflec- 

 tion are similar ; but it is not so in all ; a variety of fluor 

 spar, which, while it transmits green light, reflects blue, 

 and the precious opal, are striking instances to the con- 

 trary. Some glasses^ which transmit yellow light have 

 the singular power of dispersing blue rays from one sur- 



times very vivid ; "but the gayest combinations of colour are seen 

 in the littoral zone, as well as the most brilliant whites. 



" The animals of Testacea, and the Eadiata of the higher zones, 

 are much more brilliantly coloured than those of the lower, where 

 they are usually white, whatever the hue of the shell may be. 

 Thus the genus Trochus is an example of a group of forms mostly 

 presenting the most brilliant hues both of shell and animal ; but 

 whilst the animals of such species as inhabit the littoral zone are 

 gaily chequered with many vivid hues, those of the greater depth, 

 though their shells are almost as brightly covered as the coverings 

 of their allies nearer the surface, have their animals, for the most 

 part, of a uniform yellow or reddish hue, or else entirely white. 

 The 'chief cause of this increase of intensity of colour as we ascend,. 

 is, doubtless, the increased amount of light above a certain depth." 

 --p. 172. 



