M. W. Beetz on the Colour of Water. 221 



mitted ; they are reflected indeed from foreign substances in the 

 water, especially from the sea-bottom. The farther distant this 

 is, that is, the deeper the sea at the given place, the deeper will 

 be the colour of the water — deep green when the water has a 

 green, deep blue when it is blue (in transmitted light). The 

 rays which fell from above into the bell must also show the 

 colour of water, but to a much smaller extent, because the layer of 

 water which they traverse is, in any case, much less thick than 

 that which the rays coming from below have traversed. Thus 

 the upper rays brought comparatively more white light than the 

 lower ones ; and hence the upper surface of the hand had the 

 complementary colour, that is, rose-red, for the same reason for 

 which, in the blue grotto at Capri, the contrast colour, orange, 

 occurs. 



Arago adduces no experiment in support of his view ; he only 

 proposes to make one, to which reference will afterwards be 

 made. He introduces his view with the words " the reflected 

 colour of water is blue, the transmitted, as some think, green ;" 

 and upon this supposition he bases the explanation of some 

 phenomena. He shows, in particular, why the waves of the blue 

 sea are green. He considers them as water prisms, on one sur- 

 face of which the white daylight is reflected, sent through the 

 following wave, and thereby made green. But it is easy to see 

 that in the green waves, as well as in the large blue mass of 

 water, it is only a question of transmitted light. On looking at 

 the mirror-like surface of the Aehensee in a perfect calm, the 

 colour is seen to change from a deep blue in the middle to a 

 bright green, and thence into a yellowish red. This water, which 

 contains very small quantities of humus salts, colours the light 

 greenish when it only passes through thin layers, and blue 

 when it passes through thicker. This phenomenon has many 

 analogies. Newton says *, it must be noticed that in coloured 

 liquids the colour alters with the thickness. For instance, a red 

 liquid in a conical glass held between the light and the eye 

 appears pale yellow near the bottom, where it is thinnest, some- 

 what higher, where it is thicker, of a golden yellow, where it 

 is still thicker, red, and where it is thickest, dark red. Hence it 

 must be assumed that such a liquid absorbs the violet and indigo 

 rays very readily, the blue rays with greater difficulty, the green 

 ones with still greater difficulty, and the red ones most of all. 



This is just the case with bluish-green sea- water. It absorbs 

 the red rays very easily, the green ones with more difficulty, and 

 the blue ones most of all. Hence when white daylight passes 

 to the bottom through a thin layer of this water, and reflected 

 from this returns to the air, it is feebly green. If on both 



* Loc. cit. 



