On the Colour of the Atmosphere and Deep Water. 357 

 sufficient, however, to shew the inequalities of its surface to a 

 considerable depth, while above the water it is very obscure. 

 The line of the level of the sea in contact with the rock is like- 

 wise well marked, and bears some resemblance to the phenome- 

 non of the oars, which were luminous when in the water and 

 dark above its surface ; but here the feeble light has a yellowish 

 tinge, instead of being bright and blue as in that instance. The 

 depth seems to increase according to the degree of attention with 

 which it is observed, and the bottom is soon discovered, al- 

 though in this place it is at the depth of forty feet. The white 

 plate which I sank was easily distinguishable on the dark sand. 

 Its colour, instead of being green, as in the experiment made in 

 the sunshine, was slightly yellow. 



The feeble yellow light which illuminates the submarine wall, 

 in this part pf the grotto, is derived from the reflexion of the 

 bottom, and from that part of the opposite wall which receives 

 the light from without. This light, which has traversed a great 

 mass of water, must be yellow, like that which is transmitted 

 by opaline liquors ; thus the opaline property of the sea explains, 

 in a satisfactory manner, the principal phenomena to which the 

 particular construction of the grotto gives rise. I have attempt- 

 ed to give an idea of this construction, by means of the subjoined 

 figure, which represents the exterior rock in the sea, and above 

 the surface. '"^ ^'"'"^ ' '^^ "[ '^^''^''' ''^'''''^^''^ ^'^^ '^^* ' 



i i ar\ h h 





r-f^p^iftei on ? 



1 \ 



I n i ni i 



^ 



The small entrance is at a, above the level of the water, which 

 is represented by the line bhb- The eastern side of this entrance 

 is continued almost perpendicularly downwards" for SO or^40 

 feet; there it appears to be cut horizontally 2X d\d^ and sus- 

 pended in the dark blue waters of the sea; c^ ^ c is the supposed 

 continuation of the eastern side of the entrance to the bottom, 

 which, as has been seen, is 67 feet in depth. The western side 

 of the entrance yyy* forms an angle of 10 or 1^ feet deep; 

 it then continues in a horizontal direction for 20 or 25 feet, af- 



