PHYSICAL PROPERTIES OF SEA WATER 27 



The numerical value of the absorption coefficient depends upon the unit 

 of length in which L is expressed. In physics the unit is 1 cm, but in 

 oceanography it has become common practice to use 1 m as the unit of 

 length. Therefore, the numerical values of the coefficients that will be 

 given here are one hundred times larger than the corresponding values 

 given in textbooks of physics. 



The decrease of intensity of radiation passing through a layer of 

 water depends not only upon the amount of radiation which is truly 

 absorbed — that is, converted into another form of energy — but also 

 upon the amount which is scattered laterally. In ''pure" water the 

 scattering takes place against the water molecules, and the effect of 

 scattering is related to the molecular structure of the water. However, 

 when measuring the absorption in pure water, the effect of scattering is 

 not separated but is included in the absorption coefficient, which varies 

 greatly with wave length. 



Extinction Coefficients in the Sea. In oceanography the greater 

 interest is attached to the rate at which downward-traveling radiation 

 decreases. The rate of decrease can be defined by means of a coefficient 

 similar to the absorption coefficient: 



Kx = 2.30 (log Ix.. - log /x.a+i), (II, 12) 



where I\.z and /x.z+i represent the radiation intensities of wave length X 

 on horizontal surfaces at the depths z and (^ + 1) meters. Different 

 names have been proposed for this coefficient, such as 'Hransmissive 

 exponent" or ''extinction coefficient." The latter name has been widely 

 used and will be employed here, although the process by which the 

 intensity of radiation is reduced will be called absorption. The absorp- 

 tion of radiation in the sea is complicated by the increased scattering due 

 to suspended particles and by the presence of dissolved colored substances. 

 The extinction coefficient of radiation of a given wave length therefore 

 varies within wide limits from one locality to another, and in a given 

 locality it varies with depth and time. 



The first crude measurements of absorption in the visible part of the 

 spectrum were made by lowering a white disk of standard size (diameter, 

 30 cm), the Secchi disk, and observing the depth at which it disappeared 

 from sight. Comparisons with recent exact measurements have shown 

 that the extinction coefficient of visible rays can roughly be obtained 

 from the formula k = 1.7/D, where D is the maximum depth of visibility 

 in meters, as determined by the Secchi disk. 



The next step in the investigation of the absorption of radiation in 

 sea water was made by subsurface exposure of photographic plates 

 enclosed in watertight containers. Such experiments, which were con- 

 ducted by Helland-Hansen on the Michael Sars Expedition by exposing 

 panchromatic plates at different depths in the vicinity of the Azores, 



