Shelford and Gail (1922) first used photo-electric cells for submarine photometry in the Puget Sound. Poole (1925) produced an apparatus which was suitable for use on small ships in the open sea. This apparatus was used near Plymouth by Poole and Atkins (1926, 1928, 1929) with various minor alterations in design which rendered it more reliable and less subject to errors due to electrical leakage. The object of these researches was to study illumination in relation to photosynthesis, but Russell perceived their bearing on his own work on the phototropic movements of the zooplankton. When considering the research programme of the Atlantis, Dr. George L. Clarke decided that it would be of value to carry out photo-electric measurements of submarine illuminations in connection with his work on vertical distribution of the plankton. A request was accordingly made by Dr. H. B. Bigelow, Director of the Wood's Hole Oceanographic Institute, that such an apparatus should be assembled under our direction. The apparatus here described has been constructed closely upon the lines of H. H. Poole's original apparatus as modified in 1928. Special attention, however, has been given to the elimination of electrical leakage. Furthermore, J. H. J. Poole (1928) devised an apparatus in which rate of flash of a neon lamp was used to measure light. This rate of flash method is one of integration and not an instantaneous measurement as is that of H. H. Poole. The value of the neon lamp method lies in the fact that light under the water is extremely variable near the surface owing to the motion of the waves. This method of measuring under-water illumination was used in Lough Bray, Co. Dublin, by J. H. J. and H. H. Poole (1930), and has since been used at sea by Poole and Atkins (1931). It seemed advisable to include a neon lamp photometer in the outfit so that errors in determining the loss of light on entering the water might be minimised.