During a cruise in the southern North Sea and English Channel in 1948 a series of measurements on light penetration was secured.

The apparatus used has been described, and methods of handling it discussed. A description of the experiments for determining the transmission curve of the niters used in the photometers, has been included.

Of the systematic and random errors to which the photometer measurements were liable the most serious lay in not knowing the precise depth of the sea cell.

Extinction coefficients have been determined from the gradients of logarithmic transmission-ratio/depth graphs, and the results tabulated for each station.

Temperature gradients were never large, usually of the order of 0·02 degrees per metre, the temperature usually dropping with increased depth, frequently there was no gradient at all.