Having made several experiments with the Drummond light, and other means of illuminating light-houses, undertaken at the request of the Turkish Government, with a view to placing lights at the entrance of the Bosphorus from the Black Sea, I have been led to observe some facts regarding the illuminating powers of the lights themselves, and the increase obtained by the use of reflectors and lenses, which, I trust, may not be found uninteresting. On the increase of illuminating power obtained by Lenses and Reflectors . Let L in the annexed figure represent a lamp; m, m , two reflectors, which may be so adjusted as to throw the reflected images either in parallel lines on a screen at P' and P", or at such an inclination as to unite with that of the light itself at the centre point P. Let also s s represent a screen of such imperfect transparency as to absorb the same quantity of light in transmission as the mirrors m, m absorb by reflection; then in the first case the three images P', P, P" will have equal surfaces and intensities*, and the illuminating power will be three times that of the central lamp; and when by a different adjustment of the mirrors the three images are blended in one, then the surface will be equal to that of the central image, but the intensity three times greater, so that in either case the illuminating power will be proportional to the number of mirrors, or to the surface of those mirrors. If, therefore, we conceive the whole space between m, m to be filled with mirrors, to reflect the light in parallel lines on the screen P', P, P'', it is clear that the illuminating power of the lens will be expressed by the number of times the surface of the central image is contained in the whole surface of the screen P', P"; and this is true whether we consider the several images to be thrown in parallel lines, or condensed in a focus, or dispersed over a larger surface, for as the illuminated surface is contracted, the intensity is increased, and as it is extended, the intensity is diminished in the same proportion, so that under all circumstances the product of surface and intensity will be a constant quantity. Hence the illuminating power (abstracting from absorption) will be increased by the reflector in the ratio of the surface of the lights to the surface of the end or section of the reflector. Or in other words, the area of the end of the reflector divided by the area of the light, will be a numerical measure of the illuminating power.