354 On the Colour qf the Atmosphere and Deep Water. 



blue, because its colour will not be affected by the yellow reflec- 

 tion from the bottom ; this may take place in deep rivers^ when 

 their banks throw a shade on the bottom. 



Thus, the depth of the bottom, and its colour, may occasion 

 variations in the colour of the sea ; and it may be regarded as 

 an established fact, that when the light of the sun, transmitted 

 by the waters, is not lost in their depth, but reflected from the 

 bottom, the sea will assume a tint of green. 



In the high seas this effect may be produced by beds of sub- 

 marine plants, or by those myriads of microscopic molluscae 

 which often cover a vast Space, and which may act on the lighty 

 or even possess in the mass a slight permanent colour. ->- ^^ ' 



Colours transmitted by deep waters cannot be observed di- 

 rectly, like those of the air, which are depicted in the clouds. 

 !A single observation is recorded on this subject. The learned 

 Halley, having descended into the sea in a diving-bell, observed 

 that a ray of the sun which penetrated to him across an opening, 

 closed with glass, tinged the back of his hand with rose-colour. 

 The experiment would have been more conclusive for the prob- 

 lem to be solved, had the ray fallen on a white surface ; the red- 

 ^dish hue of the hand must necessarily have had an influence on 

 the observed colour. It is not probable that he had descended 

 inore than 30 or 40 feet ; but at this depth the colour of the 

 transmitted light could only be a scarcely perceptible shade of 

 yellow, mingled with shades of white, and this, in connexion 

 with the colour of the hand, might appear to be a rose tint. 

 Halley observed that the under side of his hand was of a green 

 colour, which was rib doubt owing to the reflexion of the bottom. 

 The contrast of this green light, with which he was surround- 

 ed, with the colour produced by the isolated ray on his hands, 

 , might contribute to th^ illusion which I suppose^ hirti to have 

 ^^'been under. 



^'^*- The cause of the greenish-blue colour of the crevices in the 



"^^Ia6ieirs'id the same as that which renders the waters green near 



•'*^the shore. If the mass of the glacier were as large and homo- 



' igeneous as that of the sea^ the interior of the crevices would be 



blue ; but the ice contains bubbles of air, particles of snow, and 



fissures, which reflect the light, and throw it from one face to 



another of the crevice, till it is transmitted to the opening; these 



