Published online by Cambridge University Press: 11 May 2009
1. Measurements of the penetration of light of various colours into the sea were carried out by means of a selenium rectifier cell. The mean vertical absorption coefficients were: for blue, 0·177; green, 0·153; “white,” viz. no filter, 0·216; yellow, 0·243; red, 0·345. The results accord well with those obtained under similar conditions with emission cells and show that, for water a couple of miles offshore, green light penetrates best.
2. The current was measured using the potentiometer-telephone method as for emission cells, but determining the drop in potential across a low resistance, 10,100, or 1000 ohms. The motion of the ship was too violent to permit of the use of any available galvanometer of adequate sensitivity.
3. Rectifier cells are more convenient to use than emission cells, as the currents to be measured are much larger and the absence, of high potentials greatly simplifies all insulation problems. On the other hand the curvature of the illumination–current relation involves additional labour in standardizing and in calculating results.