A Signal from a marine light is the oldest and simplest of the means available to the navigator for determining his position: so old, indeed, that one may well wonder whether anything can still be added to our knowledge of it, and so simple that one may question, too, whether anything is to be gained by seeking to perfect it while such vast fields of inquiry still await imaginative scientific research. However, like all human activities, navigation is caught up in a rapid evolutionary process which calls for an adaptation of time-honoured methods to new conditions of operation. Moreover, whatever advantages modern methods of position-finding may offer, the marine light will always have one great advantage that cannot be replaced, that of being directly perceptible to the human eye. It is well worth our while, then, to have a look at any defects in the present system of marine lights and see how they may be put right.