photo-current and an illumination for an outer resistance smaller than 

 100 ohms, some of the cells used in the present investigations had shown 

 a discrepancy from the linear relation. For such cells the following formula 

 has been used to calculate the intensity of illumination from the reading 

 ■of micro-ammeter — 



log L = {a + bl) log I, 

 where L is an intensity of illumination, I is a reading of micro-ammeter, 

 and a and b are constants obtained by an experiment for each cell. 

 Generally b is verj^ small and we are able to regard {a + bl) as a constant, 

 then we can calculate the \'ertical extinction coefficient of sea-water 

 by the following formula — 



k = 2-303 {a + bl) (log I^ - log I,) / (D, - D.) 

 where D^ and D^ are depths at which under- water illuminations are 

 measured and the intensities of illuminations at the depths are I;^ and 

 I, respectively, and ^ is a vertical extinction coefficient of sea-water. 

 For most of photo-cell a is unity and b vanishes, then the above formula 

 becomes — 



k = 2-303 (log Ii - log lo) / (D, - Do) 



3. Results of Measurement 



The present investigations have been carried out from 1940 to 1944. 

 Localities were restricted to the coastal waters of north-eastern part 

 of Japan, which are illustrated in Fig. 2. 



At most stations under-water illuminations were measured for 

 three parts of spectrum, red, green, and violet light. The relations 

 laetween the vertical extinction coefficients for these three bands of 

 spectrum are illustrated in Fig. 3. In this figure the results obtained 

 by other authors for the similar bands of spectrum at various localities 

 are also reproduced simultaneously. It is noticeable from the figure 

 that the relations between the vertical extinction coefficients for the 

 ■different bands of spectrum are rather scattered, accompanied with the 

 -change of season as well as of localities. As an average relation between 

 them we get following formulae, which are also illustrated in the figure 

 by straight lines — 



kv = 4-8 yfegl-5, 

 and kr: — 0-89^g0-5, when kr, is smaller than 0-2, 



^r — l-25^gO-8, when kg is greater than 0-2, 



where kv, kg, and ki are the vertical extinction coefficients for violet 

 green and red light respectively. The relations between the violet and 

 green Hght are expressible by a single function. Between red and green 

 light, however, it must be expressed by two different functions. Following 

 conclusions are supposed from this fact : seston in sea-water equally 

 increase the vertical extinction coefficients for green and violet light, 

 iDut for red light they are less effective than for green and violet 

 light so long as their total amounts are not enormous. In the 

 very turbid sea-water, when the vertical extinction coefficient for green 

 light exceeds 0-2 — about ten times greater than for pure water — -the 

 effects of seston upon the vertical extinction coefficient for red light 

 increase as well as for green and violet light, then a new relation between 

 them appears. 



The results of measurements are classified according to the colour 

 of sea in Forell's scale. In the table, a result of measurement at the 

 stations where more than five filters are used simultaneously are 



299 



