1. A description is given of some measurements of the penetration of light into sea-water carried out near Plymouth in the autumn of 1925 by means of photo-electric cells, the current through the cell being, measured by a method, previously described, which is not affected by the motion of the ship.

2. The absorption coefficients found for off-shore waters down to 30 metres are smaller than the average of those found by several recent observers in various localities, but considerably larger than those found by Shelford and Gail for the deeper waters of Puget Sound. The coefficients tend.to increase with increase of depth, and are generally larger for in-shore waters. The clearest water, 20 miles out in the English Channel, gave an absorption coefficient 0-110 for the upper 10 metres, 0-117 for the second, and 0-133 for the third. This water, with glassy surface, transmitted 0-54% of the vertical illumination to 34-8 metres, 28-3% to 8-9 metres, at which depth a white disc was just visible, and 71-2% to 1-5 metres.