The theory of the Secchi disk given by Shoulejkin1 is discussed and a correct expression derived for the depth to which the disk, painted white, should be sunk in an ocean of pure dust-free water so as to be just visible. With ordinary paint the maximum depth is computed to be about 120 meters, whereas the greatest observed depth in the ocean is about 60 meters. The difference is due (1) to the presence of suspended matter and (2) to a possible extra absorption in the blue and violet. Contrary to the opinion expressed by Shoulejkin, it is shown that in the blue region the brightness due to molecular scattering alone would be appreciable, in fact about one-sixth that of blue sky. This is confirmed by rough observations made in the Bay of Bengal.