TRANSLATIONS AND SELECTED ARTICLES. 45 
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 pro- 
poses 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 supposi- 
tion 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 surface 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 Achensee 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 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 ; somewhat 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 least 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 least 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 courses it has traversed great 
distances, it is blue. It also appears green when it has passed through 
the moderately thin section of a wave (which it may indeed have 
reached by reflexion from another wave.) 
I spoke just now of the reddish-yellow colour in the almost dry 
