Recently Jones & Wills (1956) have related the attenuation of light in the sea and in estuarine waters to the concentration of suspended solid matter. The attenuation was measured with an in situ hydrophotometer with a beam acceptance angle of approximately 3.20°. The concentration of suspended materials was determined by filtering in the case of natural samples or by adding known weights of kaolin or mud to tap water. In the discussion of their data and methods they have made application of the difFraction theory for opaque particles relatively large compared to the wave-length of light.