M. Arago 07i ike Colour of the Ocean. 381 



floating upon the surface, and so hiding the natural colour of 

 the liquid. The zones of carmine red, which several voyagers 

 have traversed on the great ocean, arise from the same cause. 

 In Switzerland, according to Sir H. Davy, when the tint of 

 the lake passes from blue to green, it is because its waters are 

 impregnated with vegetable substances. Finally, near the 

 mouths of great rivers, the sea has often a brownish hue, aris- 

 ing from the mud and other terrestrial matters, which are there 

 lield in suspension. We have thought it right thus to insist 

 upon the colours produced by foreign matter mixed with the 

 water, so that they may in no degree be confounded with those 

 upon which we are yet to dwell. 



The celestial blue tint of the sea is modified, and sometimes 

 even totally changed, in those localities where the water is not 

 very deep. It is, then, because the light reflected by the bot- 

 tom reaches the eye mixed with the natural light of the water. 

 The effect of this superposition may be calculated by the laws 

 of optics ; but we must join to our acquaintance with the na- 

 ture of the two commingled tints, that, which is more difficult 

 to ascertain, of their comparative intensities. Thus, a bottom 

 of yellow sand but lightly reflecting, gives to the sea a green 

 tint, because the yellow mixed with the blue, as every one 

 knows, produces a green ; now, without changing the shades, 

 if you replace the dull yellow with a bright yellow, the slight 

 blue of pure water will scarcely make this a lively light green, 

 and the sea will appear yellow. In the Bay of Loango, the 

 waters are always deep red; so much so, that it is said they 

 are mixed with blood: and Captain Tuckey satisfied himself 

 that the bottom is intensely red. Let us substitute for this 

 bright red bottom, one of the same shade, but obscure and 

 slightly reflecting, and the waters of the Bay of Loango would 

 then appear orange-coloured^ or even perhaps yeUow. 



Against this method of regarding the subject, one objection 

 is made which, at first glance, appears serious: a bottom of 

 white sand, it is said, ought not to alter the hue of the sea, for 

 if white enfeebles the colours with which it mixes, at all events 

 it does not change the tint. But there is a ready answer to this 

 objection. For how can we be certain that the sand at the bot- 

 tom is white ? It is not in the open day, after we have fished 



