Color of the Air and of Deep Waters. 69 



We thus perceive that the light of the sun transmitted through wa- 

 ter and reflected from a white surface, produces green. The cause 

 may be readily conceived by admitting in deep waters, the same 

 opaline property which we recognize in air. The light, after pene- 

 trating a mass of one hundred feet of water to reach the plate and 

 return to the surface ought to be yellow like that which would be 

 transmitted by an opaline fluid ; this color reflected by the plate, mix- 

 ed with the blue which reaches the eye from all quarters, produces 

 the green. If the bottom of the sea were white, like ceruse, the 

 waters near the shore would present the same green tint which the 

 plate produced at different depths ; but the bottom is generally of a 

 dark grey which reflects less light, and therefore yields only a dark 

 and uncertain green ; hence the green color of the sea as witnessed 

 near the shore is owing to the reflexion of light from its bottom. To 

 leave no room for doubt in this matter, I took a boat and pushed out 

 from the shore, under a clear July sun, at eleven in the morning to 

 examine the changes which might be perceptible in the color of the 

 water, viewed on the side of the boat opposite to the sun. 



At fifty toises from the shore, the water was decidedly green, the 

 shade of which remained during fifteen minutes; it then became a 

 bluish green, and in advancing, the blue continued to increase and at 

 length to predominate, and in an hour's time the water under the boat 

 was of a pure blue without a mixture of green. 



In returning to the shore I was attentive to the reappearance of the 

 green and as soon as I found it clearly marked, I sounded and found 

 the depth one hundred and fifty feet ; thus the light which renders 

 the sea of a green color, passes through three hundred feet of water. 

 But in that part of the gulph, another cause contributed to the green 

 color, viz. the impurity of the water as it exists to the extent of some 

 miles from the shore. The bay of Naples receives no river that 

 can give motion to the waters charged with all the filth of that pop- 

 ulous city. On the shores of the Isle of Capri, the water is perfectly 

 blue at eighty feet, while near Naples it requires one hundred and fif- 

 ty feet, a difference which must be ascribed to the impurity of the 

 water in the bav. 



4 



If the bottom be of a black or very dark color, the water may be 

 blue at a much less depth than eighty feet. Besides, if an obstacle 

 intercept the direct rays of the sun, so as to throw a shade over the 

 bottom, while the water itself is illuminated, the latter will be blue, 

 because no longer colored by yellow rays from the bottom j this ef- 



