46 TRANSLATIONS AND SELECTED ARTICLES. 
places which has been noticed by so many observers. This colour 
depends entirely on the nature of the ground. Most frequently it 
consists of whitish sand, or whitish pebbles. If these were absolutely 
white, if they reflected colours ina diffused manner to the same ex- 
tent, the reddish colour would not occur. A new porous clay-cell of a 
Grove’s battery may appear quite white, while, when it is moistened 
with water, it is of a rusty yellow or flesh-red colour. Hence its sur- 
face acquires the property of reflecting red light to a preponderating 
extent. If, now, the substances which constitute the sea-bottom have 
the same property, the bottom will appear red in those parts in which 
it is covered with quite thin layers of water. If the thickness of the 
layer of water increases, fewer red rays reach the bottom ; the return- 
-ing rays are again partially absorbed by the water, and thus the red 
colour is continually disappearing, although the forms of substances 
lying on the ground can always be distinctly perceived. 
Moreover, this red colour is much increased by contrast. In the 
dry places of the AarI have often observed that the bright red, 
which they show, diminishes considerably when they are viewed, not 
near the beantiful green of the deeper water, but through an isolated 
tube. 
There might seem to be a fact in discordance with the state- 
ment, that sea-water m thin layers is green, and blue in thicker; a 
white object, for instance an oar, appears of a distinctly pure blue 
‘when immersed at even a very inconsiderable depth below the level of 
‘the Achensee, while it is of an intense green below the Tegern-or 
KGnigsee. The light which impinges upon the white surface of the 
oar, has had to traverse a much more considerable distance than that 
from the surface of the water ; it comes from the side through a con- 
siderable mass of water, in which it has assumed the characteristic 
colour of the lake. But if the same white surface is brought near 
the bank and turned towards it, and is at the same depth as in the 
former case, it is seen in the Achensee to be of an almost unaltered 
white, while in the Tegernsee it is always somewhat greenish; for the 
colour of the blue water is only perceptible at great distances, that of 
the green at very small ones. : 
This surprising strong coloration in consequence of laterally inci- 
dent light, led me to the proposal which Arago has made, to investi- 
gate the true colour of water in transmitted light. 
A hollow prism made of glass plates is so brought under water that 
