Three methods of determining extinction coefficients of light in sea water have been compared: (a) on water samples contained in a Pulfrich photometer, (b) from brightness profiles determined by aerial photographic survey, and (c) by photoelectric underwater measurements. The ratios of extinction coefficients for green and red light as determined in 1945 by the Pulfrich photometer and by the brightness profile method are given in Table 3, columns 9 and 10. For green light the ratio is about i-8 and for red light 1-3.

The departure from unity is attributed to the geometry within a confined tube and, within the Pulfrich measuring assembly, of light rays scattered forward by particles present in sea-water samples. In the following year, 1946, comparisons were made between the Pulfrich method and the submarine photometric method of Atkins and Poole. The discrepancies were variable and even larger, the ratios averaging 3-8 for green light and 2-0 for red light.

The brightness profile and submarine photometric methods have not been directly compared, but, by comparing the two sets of determinations against the Pulfrich photometer, it would seem that extinction coefficients determined with the brightness profile method are about 2-2 times those determined by the submarine photometer in green light and about 15 times in red light.