cell resembles that of the human eye, but is somewhat more sensitive 

 for the ultraviolet and infrared regions. The sensitivity of the cell 

 for white light is about 0-5 microamperes per lux ; the sensitivity had, 

 however, been measured for each cell before the measurement of 

 under-water illuminations comparing with a standard photo-cell provided 

 by the same company. 



The photo-cell is put in a bronze casing with a window covered by 

 a thick glass, the thickness of the glass is 5 mm. and the aperture of 

 window is 60 mm. The photo-cell is connected to instruments on 

 board a boat bj^ means of a two-core cable covered with rubber sheath. 

 Above the thick glass an opal glass is laid, the upper surface of the 

 opal glass is adjusted to be at the same plane to the surface of frame 

 clamping the window glass, so as to introduce the whole light rays 

 coming from the upper hemi-sphere under the sea. 



Between the opal glass and the window glass a light-filter is inserted 

 in order to Umit the under-water illumination in a narrow band of 

 spectrum. The light -filters used in the present investigations are manu- 

 factured by the Tokyo Electric Co. The spectral response of the photo- 

 cell with the filters are also illustrated in Fig. 1. The optical centre of 

 spectral response for each filter is shown by a short arrow in the figure. 

 The wave-lengths of them are 665, 645, 615, 570, 535, 495 and 405^ 

 for the filters V-Rl, V-R2, V-R3, V-01, V-Gl, V-Bl, and V-Vl 

 respectively. 



lOO 



SO 



o 



700 



60O 



SOO 400 



li/a-ve -length in jx 



Fig. 1. — Spectral sensitivity of Mazda photo-cell and the spectral response of the 



cell with filters 



A photo-current from the under-water photo-cell is measured b}' 

 means of an ammeter installed on hoard a boat. Between the cell and 

 the ammeter a key- and resistance-box is inserted, which contains shunt 

 resistances for the ammeter to alter a range of measurement, as well 

 as a switch to connect the ammeter to the under-water photo-cell and 

 a cell laid horizontally on a deck of boat alternatively. The internal 

 resistance of ammeter is 50 ohms, and the full scale is 150 micro-amperes. 

 Although a Mazda photo-cell generally has a linear relation between a 



298 



