352 On the Colour of the Atmosphere and Deep Water. 



rent. The green tint often assumed by the waters of the sea, 

 may seem to render it doubtful whether the property of reflect- 

 ing blue is inherent in the nature of water ; but this green hue 

 is never observed but when the sea is not of sufficient depth, 

 that is to say, when the bottom reflects the transmitted light. 



On looking at the sea from an eminence of about fifty toises, 

 on the shore of the island of Capri, I observed some portions 

 which were of a more beautiful green and greater briUiancy 

 than the surrounding water ; to ascertain the cause, I went to 

 the spot in a boat. On reaching the margin of the sea, these 

 portions were no longer distinguishable, but they soon reap- 

 peared, and I perceived that the colour was produced by white 

 rocks, which were easily seen notwithstanding the great depth, 

 as they lay on a bed of dark sand ; the rocks, thus seen in a ver- 

 tical direction, were of a less intense green than they appeared 

 to be from above, but I could not doubt that they were the 

 cause of the phenomenon in question. 



In order to assure myself of this by direct experiment, I pre- 

 pated a square plate of white-iron, 14 English inches on the 

 side, painted with white lead. Having suspended this horizon- 

 tally to a line, I sunk it in a deep part of the sea where the 

 water under my boat was blue without any mixture of green, 

 following it with my eyes under the shade of an umbrella held 

 over my head. At the depth of 25 feet, it had taken a very 

 perceptible tinge of green ; this colour gradually became darker 

 to the depth of 40 feet, when it was of a beautiful green inclin- 

 ing to yellow ; at 60 feet the tint was the same, but of a deeper 

 shade ; the form of the plate was now no longer distinguishable, 

 and at 80 feet I saw only a greenish glimmer which soon disap- 

 peared. 



We see, therefore, that the light of the sun, transmitted by 

 the water of the sea and reflected by a white surface, produces 

 a green colour. The cause of this is easily perceived, by ad- 

 mitting the existence of the same opaline property in deep water 

 that is found in the air. The light, after having penetrated a 

 mass of water of 100 feet to reach the plate and return to the 

 surface, must appear yellow like that transmitted by an opaline 

 liquor ; this colour reflected by the plate, mingling with the 

 blue of the interior, produces the green colour. If the bottom 



