66 



THE BOTTOM OF THE SEA. 



and is seen from a great distance, by its reflected light, 

 through an imperfectly diaphanous medium. 



To appreciate correctly the colour of the sea-bottom, 

 we must be on our guard against a very common illu- 

 sion. Why should a white sand change the colour of 

 the watei-, or affect it otherwise than with a slight 

 tint of white ? To answer this question, we must re- 



Fig. 14. — Incidence of the Rays of Light on a Cahn Sea. 



member that sand is white when it is taken out of the 

 water and dried. If we throw upon it rays of red, or 

 green, or any other coloured light, the sand will appear 

 of that colour. Water seen by reflection appears 

 blue ; seen by transparency, it appears green. As 

 the sand at the bottom of the sea receives a green 



