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 sibunine 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. Ihave 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. 
The little entrance is shown at a above the level of the sea repre- 
sented by the line 6b. ‘The eastern side of this entrance extends 
almost perpendicularly to the depth of thirty or forty feet, when it ap- 
