1. Measurements of the under-water illumination were made in eight lakes by employing Mazda selenium rectifier photo-electric cell and an unipivot pointer galvanometer.
2. Within the limit from 5 to 12 m. in transparency, the illumi-nation at the depth at which the SFCCHI'S disc was just visible was 12.5-18.5% of that at the depth of 1 cm. below the surface, the mean of which was 14.5%.
3. The following relation was observed between the transparency (t) and the absorption coefficient (k) of water per meter: t=1.9/k
