68 Color of the Air and of Deep Waters. 
reflexion, disengaged, as much as possible under an open sky, from 
the reflexion at the surface. 
Agitation of the surface has a great effect on the color. A tran- 
quil sea sometimes reflects the warm color of the horizon, represent- 
ing all the tints of a luminous sky so exactly, that the sky and sea ap- 
pear to be blended with each other; but if a gentle breeze ruffles the 
surface, the brilliant tints vanish, and the blue from the interior im- 
mediately predominates. 
Such is also the cause which enables one to distinguish the course 
of the Rhone far into the waters of the Leman: the progressive mo- 
tion of the river in the motionless water of the Lake produces an 
agitation which diminishes the brilliant reflexion of the sky and ren- 
ders the color of the water more sensible. 
The green tint which the sea often assumes, may seem to throw 
some doubt on this property of reflecting the blue ray, regarded as 
inherent in the nature of water; but this green color is observable 
only when the depth of the sea is insufficient, that is when the bottom 
may reflect the transmitted light. 
In looking at the sea from an elevation of about fifty toises, on the 
shore of the Island of Capri, I observed spots which were of the fin- 
est green, much more luminous than the dark blue sea, with which 
they were surrounded. ‘To ascertain the cause, I took a boat and 
proceeded to the place. The spots then were no longer perceptible, 
but I soon rediscovered them, and found that the color was occasion- 
ed by white rocks which were easily distinguished, notwithstanding 
their great depth, from the dark sandy bottom in which they repo- 
sed. These rocks, viewed in a vertical direction, were of a lighter 
green than when seen from the height, but I could not doubt that they 
were the cause of the phenomenon. 
To settle the point by direct experiments, I prepared a square 
sheet of tinned iron, fourteen inches long, painted it white on one 
side, suspended it horizontally to a cord, and sunk it in a deep place, 
where the water under the boat, was blue without any mixture of 
green, watching the effect under the shade of an umbrella which was 
held over my head. Atihe depth of twenty five feet, it acquired a 
very sensible green tinge, and this color became more and more in- 
tense to the depth of forty feet when it was of a beautiful green, in- 
clining to yellow; at sixty feet the color was the same, but of a dark- 
er shade, and the square figure of the plate was no longer distinguisha- 
bie; until at eighty feet, there was apparent only an uncertain glim- 
mering of green which soon disappeared. 
