332 M. Arago on the Colour of the Ocean. 



up a portion, and so exposed it to the white light of the sun, 

 or the clouds ! Is the sand in the same condition when be- 

 neath the water ? If in the open air you were to illuminate it 

 with red, green, or blue light, it would then appear red, green, 

 or blue. We have then still to inquire what colour strikes it 

 at the bottom of the water ? 



Water is in the condition of many other bodies, which phi- 

 losophers, chemists, and mineralogists, have very deeply studied, 

 and which possess two kinds of colours, — a certain colour which 

 is transmitted, and another colour, quite different from the first, 

 which is reflected. Water appears of a blue colour, by reflec- 

 tion ; and some imagine it is of a green colour by transmission. 

 Thus, water disperses in all directions, after having blued it, a 

 portion of the white Hght which went to illuminate it ; this 

 dispersed light constitutes the proper colour of liquids. As to 

 the other rays, irregularly transmitted, their passage across the 

 water makes them green, and this intensely in proportion as 

 the traversed mass is thick. 



These notions being admitted, we may now return to the 

 case of a not very deep sea, with a bottom of white sand. This 

 sand receives the light only through a stratum of water ; the 

 light, then, is green when it strikes the bottom, and it is with 

 this tint it is reflected ; and in the second traject which the lu- 

 minous rays make through the same liquid in returning from 

 the sand to the open air, their green tint sometimes so predo- 

 minates, that it prevails over the blue. This, then, perhaps 

 may be the whole secret which, to the practical seaman, is, in'' 

 the time of calm, the certain and invaluable index of great 

 depths. 



We have put it, in time of calm, and not without reason ; 

 for when the ocean is agitated, the waves suitably elevated may, 

 in fact, convey to the eye so large a quantity of transmitted or 

 green rays, that the reflected blue ones shall be entirely masked. 

 A few short observations will make this evident. 



Let us imagine a triangular prism placed^ in the open air, 

 horizontally, before an observer, somewhat lower than he is. 

 This prism cannot conduct to the eye, in the way of refraction, 

 any ray coming directly from the atmosphere. On the con- 

 trary, the anterior face of the prism will throw towards the ob. 



