Polarization of scaltered sunlight in deep water 



4U 



from horizontal were correspondingly diflercnt even though ihcy do not agree 

 numerically with the angles of refraction. Note, however, that the ratios of the 

 observed angles to the calculated ones were more nearly alike. In other words, the 

 observed angles of polarization deviated significantly from those calculated simply 

 on the basis of surface refraction. This introduces the second pomt illustrated by 

 Tables I, II, and Figs. 2, 3. 



The second point is the effect of depth per se on the tilt of the polarization plane 

 independent of the solar altitude. This influence was obvious in the data of all the 

 stations deeper than 30 m. At 200 m (Table II), for example, the average tilt of the 

 polarization plane 90° from the sun's azimuth was 8", which was only 30% of the 

 angle of refraction of the sunlight at the surface when the photographs were made. 

 For the stations at intermediate depths, the tilt of the polarization plane was also 

 intermediate, being significantly less than the angle of refraction yet considerably 

 larger than the tilt found at 200 m. 



DISCUSSION 



The new facts which have been presented above are mainly of interest for two 

 reasons. One is their possible usefulness in solving physical optic problems relating 

 to underwater polarization in particular and to light penetration in general. The other 

 is their potential helpfulness in evaluating the biological importance of submarine 

 polarization patterns. 



With relation to the first point, it is interesting that two of the main findings reported 

 here were predicted on a theoretical basis before the present measurements were made 

 (Waterman, 1954 a). These were (I) that polarized light would occur down to consider- 

 able depths in the sea, probably as far as the lower limits of the photic zone, and (2) 

 that with increasing depth the influence of the sun's position on the pattern of polariza- 

 tion would gradually diminish in the upper few hundred metres after which it would 

 be negligible. Both of these effects were clearly apparent in the present data ; although 

 at the maximum depth tested, 200 m, the sun's position still had considerable effect, 

 it was only about a third of what it would have been near the surface. 



Note that these predictions were made on the assumption that Rayleigh scattenng 

 of unpolarized directional light in the water was the basis of the primary underwater 

 polarization pattern and on the currently accepted ideas about the directionality o\ 

 deep-water illumination (Whitney, 1941; Poole, 1945; Jhrlov. 1951). The tact 

 that the deep polarization as it was actually found matched the predictions m these 

 respects is in turn putative evidence for the validity of these elements in the original 



analysis. 



One might go a step further here and point out that, when the submarine polariza- 

 tion pattern itself is understood more thoroughly, instrumentation and methods like 

 those used in this work might be a convenient way to study the general problems of 



