THE PROPERTIES OF SEA-WATER 75 



Optical Characters, 



Anyone who has taken a long sea voyage will not hâve 

 failed to notice that sea-water is characterised by great clear- 

 ness and a peculiar colour. The manner in which Hght 

 pénétrâtes sea-water has a direct bearing on the environment 

 of marine organisms of ail kinds, and the two most important 

 questions for considération are the refraction and absorption 

 of light. 



Water being a denser médium than air, a ray of light 

 falling from the air into water is bent towards the perpen- 

 dicular. The index of refraction for yellow light in fresh 

 water is l'SSS, and this index increases with the salinity, but 

 decreases with the température. 



The transparency of the sea varies considerably from place 

 to place. As a gênerai rule, vessels lying at anchor in the 

 tropics can see the anchor on the bottom in depths of 10 

 fathoms and upwards, but in temperate seas this is not 

 possible, although Captain Hood in 1676 observed mussels 

 on a bottom of white sand at Nova Zembla in a depth of 

 approximately 80 fathoms (? a mistake for 80 feet). Regular 

 observations on the transparency of différent seas hâve been 

 made from time to time, the flrst of which are due to the 

 Russian scientist Kotzebue on the Rurik in the tropical waters 

 of the North Pacific Océan. He found a red cloth disappeared 

 in depths of from 1 1 to 16 fathoms, whereas a white plate was 

 visible down to 27 fathoms. The American naval ofïîcer, 

 Charles Wilkes, made numerous observations with a white 

 basin, and he noted not only the depth of disappearance, but 

 also the depth at which it again became visible when being 

 hauled in. He also noted the height of the sun during the 

 observation. In the tropical parts of the Pacific he found good 

 visibility down to depths of 16 to 32 fathoms. Observations 

 . are on the whole more successful the nearer the eye is to the 

 surface of the water. Other observations show that yellow 

 dises hâve a visibility of 88 per cent., red 77 per cent., and 



