Color of the Air and of Deep Waters. 73 
pears to be cut horizontally at dd and suspended on the dark blue 
water of the sea; decis the supposed continuation of the eastern side 
of the entrance, to the bottom, which as we have seen is sixty seven 
feet deep. The western side of the entrance fff forms an angle at 
ten or twelve feet below the surface, and is prolonged horizontally 
from twenty to twenty five feet and then descends obliquely, proba- 
bly to the bottom of the sea where it cannot be seen beyond thirty or 
forty feet. 
This construction gives an immense opening for the light to enter 
the grotto through the water, even when the little opening above the 
surface is closed, and it thus occasions, over a great mass of wa- 
ter, that dispersion of the blue ray which always takes place in deep 
and limpid waters, and which is manifested in greater splendor in 
the azure grotto in consequence of its being mingled with no oth- 
er light. 
Having considered the opaline property of air and water, let us 
now examine the production of opaline blue in opake bodies. 
The cause of the blue tint assumed by the fine skin whieh covers 
the veins has hitherto been a doubtful question. This phenomenon, 
which is uniformly connected with the opaline property of the skin, 
is mentioned by Leonard De Vinci; let us first see the conditions 
under which it exists. 
First, the vein must be deep enough to absorb all the light trans- 
mitted by the skin; and the skin must have the thinness requisite to 
transmit a great portion of the light. If the vein is thin, it reflects 
the color of the blood and becomes red ;* this color mixing with the 
opaline blue of the skin, forms those violaceous tints, observable on 
the countenances of persons of dark complexion (brouillé). If the 
vein is still thinner and nearer the epidermis, the transparency of the 
skin increases and the red color is more distinct; finally, a tissue of 
imperceptible veins, very near the surface of the skin, colors the 
cheeks and lips of young people of a fine complexion, with a uniform 
red; but we may observe that these beautiful colors have not the ex- 
act tint of the blood which produces them; it partakes of the opaline 
blue, which renders the color slightly carmine and tinges sometimes 
the lips of sanguine people of a purple or violet hue. 
*It is thus that a wide barometer tube filled with colored wine, appears black, 
while a thin thermometer tube, under like circumstances, is of a beautifal purple. 
Vou. XXVI.—No. 1. 10 
