72 Color of the Air and of Deep Waters. 



If when the observer is seated on the bench, a boat passes in front, 

 it forms no reflexion, nor shade in the water. If the eyes are then 

 covered with the hands so as to hide the boat and the water,, the 

 former appears suspended in the air like a dark silhouette crossing 

 the sky. This spectacle is so striking when first observed, that one 

 cannot avoid some apprehension on account of those who furnish the 

 occasion of it. In passing to the dark side mentioned on the right, 

 the water is no longer blue but remarkably transparent. The rock 

 below is so illuminated as to show its fissures at a considerable depth, 

 while above the water, it is very obscure. The water line is clearly 

 marked and has a yellowish tint. The depth seems to increase, the 

 longer it is observed, and at length the bottom is seen although forty 

 feet below. The white plate which I let down was very distinguish- 

 able on the darker sand. Its color instead of being green, as when 

 tried in the sun, was slightly yellow. 



The feeble yellow light which illuminates the submarine walls in 

 this part of the grotto proceeds by reflexion from the bottom, and 

 from the walls opposite, which receive the exterior light ; this light, 

 which has traversed a great mass of water, should be yellow like that 

 transmitted by opaline fluids, and thus the opaline quality of the sea 

 affords a satisfactory explanation of the principal phenomena of the 

 grotto. I have endeavored to give an idea of this construction by 

 means of the subjoining figure, which represents the exterior rock or 

 shell of the grotto as it exists both in the sea and above the surface. 



< 



c / 



L. 



\. 



The little entrance is shown at a above the level of the sea repre- 

 sented by the line b b. The eastern side of this entrance extends 

 almost perpendicularly to the depth of thirty or forty feet, when it ap- 



