Color of the Air and of Deep Waters. 73 



pears to be cut horizontally at d d and suspended on the dark blue 

 water of the sea j deck 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/// 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, probar 

 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 which 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. It 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 (brouille). 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 uniforni 

 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. 



Vol. XXVI.— No. 1. 10 



* 



