The spectral absorption of light (350–650 mμ) by pure water and bay of Fundy water was determined by means of a quartz spectrograph, Nicol prisms, and tubes of water of varying lengths up to 5 metres. The sun at noon on clear days was used as the source of radiation, its constancy being checked by means of a pyrheliometer. The tubes and spectrograph were placed in an equatorial mounting and the radiation from the sun controlled by a heliostat mirror. The results in the visible for distilled water agree with those of other workers. There appears to be a surprising difference in the absorption of ultra-violet between distilled water and natural waters. Less than 0.5 per cent of the near ultra-violet was transmitted by 1 metre of one of the samples of sea-water. A number of applications of the above method have been suggested.