THE PROPERTIES OF SEA-WATER 75 



Optical Characters. 



Anyone who has taken a long sea voyage will not have 

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

 ness and a peculiar colour. The manner in which light 

 penetrates sea-water has a direct bearing on the environment 

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

 questions for consideration are the refraction and absorption 

 of light. 



Water being a denser medium 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 i'333, and this index increases with the salinity, but 

 decreases with the temperature. 



The transparency of the sea varies considerably from place 

 to place. As a general 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 different seas have been 

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

 Russian scientist Kotzebue on the Rurik in the tropical waters 

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

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

 visible down to 27 fathoms. The American naval officer, 

 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 

 discs have a visibility of 88 per cent., red 77 per cent., and 



