SECT. 4] UNDERWATER VISIBILITY 455 



Exceptions to these rules-of-thumb are common. They seldom, for example, 

 apply to a white Secchi disk in clear water, even if the disk is observed by a 

 swimmer ; this is because of the high inherent contrast of the white disk. No 

 simple conversion exists between Secchi-disk data and the sighting ranges of 

 other objects. The nomographic charts, however, provide correct sighting 

 ranges for Secchi disks and other objects under virtually all circumstances. 



References 



Duntley, S. Q., 1949. Exploratory studies of the physical factors which influence the 



visibility of submerged objects. Proc. Armed Forces-Nat. Res. Council Vision Comm., 



23, 123. 

 Duntley, S. Q., 1950. The visibility of submerged objects I. Proc. Armed Forces-Nat. Res. 



Council Vision Comm., 27, 57. 

 Duntley, S. Q., 1951. The visibility of submerged objects II. Proc. Armed Forces-Nat. Res. 



Council Vision Comm,., 28, 60. 

 Duntley, S. Q., 1960. Improved nomographs for calculating visibility by swimmers, 



(natural light). Bureau of Ships Contract NObs-72039, Rep. 5-3, Feb. 

 Duntley, S. Q. and R. W. Preisendorfer, 1952. The visibility of submerged objects. 



Final Rep. Visibility Laboratory, N5ori 07864, Mass. Inst. Tech., Aug. 

 Preisendorfer, R. W., 1957. A model for radiance distributions in natural hydrosols. 



Scripps Institution of Oceanography, SIO Reference No. 58-42. 

 Taylor, J. H., 1960. Visual contrast thresholds for large targets. Part I. The case of low 



adapting luminances. SIO Reference No. 60-25, June 1960. 



