There is a well-known law in geometrical optics, that the “intrinsic luminosity” of the image formed by any lens system whatever is the same as the intrinsic luminosity of the object. An exception, however, which seems not to have been pointed out before, exists in the case of polarised light, based on the fact that by the agency of a double-image prism two light rays polarised in directions mutually perpendicular may be combined into one ray, which carries the total energy of both. In this way an intrinsic luminosity of image can be attained which is twice as great as that of the object.