SECT. 4] 



431 



The angular field of view of the instrument is 10°, obtained by means of an 

 optical system, and the flux is recorded by means of an RCA 931 -A multiplier 

 phototube and a microam meter. The zenith angle is fixed at 90°. 



The instrument shown in Fig. 11 was developed by Ivanoff (IvanofF and 

 Waterman, 1958), and has been used by him for measurements of the polariza- 

 tion of underwater light. The instrument is also capable of radiance measure- 

 ments in the horizontal plane and is of interest because of its simplicity and low 

 cost. The instrument uses a Westaphot photocell connected to a galvanometer. 



Fig. 11. Drawing of instrument developed by Ivanoff and Waterman (1958) for stvidies of 

 polarization of light under water. 



Flux from the horizontal direction is directed to the cell by means of a mirror 

 which scans the azimuth continuously. Azimuth position stability depends on 

 a rudder. 



Used for radiance measurements, the continuous scan will generally record 

 radiance as a wave-form, the maximum of which would locate the vertical plane 

 containing the sun. The length of the wave-form would establish the equivalent 

 of 360° in azimuth and the phase difference between two successive wave-forms 

 would be indicative of the stability of the azimuth position. 



The zenith angle of observation of this instrument is also fixed at 90°, 



4. Attenuation Coefficient 



Instruments for measuring the transmittance of a fixed path length of water 

 have been in use in oceanography for a long time. Some of these instruments 



