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The penetration of daylight to depths of hundreds of metres was measured in four regions of the eastern North Atlantic. At one of the stations the penetration of full moonlight was also observed. Attenuation curves for seven wavelengths are presented for each station, and spectra derived from these data indicate the colour of the light at the depths of observation. The spectral characteristics and intensities of light at equivalent depths in the four regions are compared, and the biological implications of differences in attenuation of light from sun and moon in a single area are considered. INSTRUMENTS Bathy-thermo-irradiance meterThis submersible instrument (Figs. 1, 2) records light and temperature and indicates the precise depth at which the records are made. It is supported in the water by a polythene-insulated, single-conductor cable along which commands and information are telemetered. The cable is wrapped on an electric winch; signals are transmitted to and from the deck unit by way of interconnected slip-rings on the winch. The system uses a sea return, and the circuit between underwater and deck units is completed by trailing a weighted conductor over the side.The light sensor is an RCA 6472 multiplier phototube selected from 40 such tubes

This is preliminary to a series of papers resulting from SOND 1965 Cruise aboard R.R.S. ‘Discovery’. The cruise was designed as an ecological study of sonic scattering layers at a certain season in a restricted volume of ocean in the region of the Canary Islands. These papers will appear periodically in this journal.

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