82 SECRETS OF EARTH AND SEA 



transmit light which has passed through a considerable 

 thickness of ice (as, for instance, in an ice cave), are deep 

 blue; there is no question of a reflection from suspended 

 particles. The green colour which some glaciers show at 

 a little distance is due to the yellow rust — iron oxide — 

 blown on to the surface of the ice and dissolved. Many- 

 glaciers or parts of glaciers are quite free from it, and of a 

 splendid indigo blue in their deeper fissures. So, too, as 

 to the sea and lakes. The Blue Grotto or Cavern of the 

 island of Capri, near Naples, is a case in point. All the 

 light which enters it comes through the sea-water and is 

 blue. I was taken to it in a boat rowed by two men. As 

 the boat enters the low mouth of the cavern you have to 

 bend down to avoid knocking your head against the rock. 

 Then you find yourself floating in a vast and lofty 

 chamber the white rocky floor of which is some twenty 

 feet below the surface of the clear water. No light enters 

 the cavern by the low part of the entrance above water. 

 Below the surface it widens and the strong Southern sun 

 shines through the clear water and its light is reflected up 

 into the cave from the bottom. It is blue, and everything 

 in the cave above as well as below the water is suffused 

 with a blue glow — a truly wonderful and fascinating 

 spectacle. In order to get the best effect you must 

 choose an hour when the sun is in a favourable position. 

 Where there is a white bottom at a depth of fifty or a 

 hundred feet, the sea has a fine ultra-marine colour, so 

 long as it is clear. It is often made green by yellow- 

 coloured impurities, either fine iron-stained sediment or by 

 minute living things in the water. The colour of the 

 water of either sea or lakes, when it is clear and overlying 

 great depths (200 fathoms and more), tends to be dark 

 indigo owing to the deficiency of reflected light. But 

 there are enough white particles as a rule to send some of 

 the light, which penetrates the water, upwards again. 



