When the French were recovering from the long night of terror, during which their commerce had been ruined and their ships disabled, they directed attention to lighthouses, and resolved to discard the very imperfect and insignificant reflectors then in use. They investigated the subject with their usual scientific skill, and the result was the invention and adoption of the system of lenses instead of reflectors, known as the Dioptric system. A transparent lens reduces to parallelism all the luminous rays which traverse it, whatever be their original amount of divergence, provided these rays proceed from a point or focus suitably situated. The substitution of glass lenses for reflectors is not a new idea, since we find that a proposal to that effect was made by a London optician to Mr. Smeaton, in 1759, for illuminating the Eddystone lighthouse, but was not adopted by him. M. Fresnel mentions that lenses had been used in England so far back as 1789, in the tower light-room at Portland Island, but from some cause or other were discontinued. On account of the great loss of light by reflexion at the surface of mirrors, the French adopted the lenses, and they soon discovered the source of failure in our use of them; they saw that, in order to render lenses superior to reflectors, the intensity of the illuminating flame must be considerably increased, as well as the size of the lenses; also, that these lenses must have a very short focus; and that, if constructed by the ordinary rules, their thickness would be great, their transparency diminished, and their weight far too great for the safety of the machinery whereby the lights were revolved. Fresnel therefore adopted the ingenious device proposed by Condorcet, that of constructing a lens of a number of distinct pieces. This method was also proposed by Dr. Brewster, in 1811. Fresnel also invented a lamp, with a number of concentric wicks, the lustre of which was twenty-five times greater than the best lamps then existing. In a lighthouse on the dioptric system, the lantern is constructed with eight sides, which form an octagonal prism around the lamp in the centre. The centre of each side is occupied by a plano-convex lens, something similar to a burning-glass, having a diameter of about fifteen inches. This central lens is not sufficient to cover the entire side. Indeed, a lens of sufficient size for the purpose would be very costly and bulky, even supposing it could be manufactured. To remedy this defect, the central lens is surrounded by a series of glass rings, the external surface of which is so formed as to have precisely the same optical effect as the great central lens. A transverse section of one of these zones or rings presents the form of a wedge, one side of which is slightly curved.