It has very justly been said that the object of placing in a lighthouse an illuminating apparatus is, that, whether it be constructed of glass or of metal, it may _bend the rays_ (which would otherwise and naturally proceed in straight lines), and illuminate a hollow sphere, so that those rays which would otherwise be thrown upon the sky, and thereby wasted, may be made to fall on points at sea, where they will be clearly visible. If the light is to be a fixed one, intended to be seen all round, and from the horizon to the base of the light-tower, the upper rays issuing from an illuminating apparatus must be directed downwards, and the lower rays upwards, so as to increase the illumination. If it is desired to light up a narrow belt of the sea, extending from the horizon to the base of the lighthouse, all the rays must be bent laterally; or they may all be concentrated and thrown upon one or more spots of larger or smaller size, according as the light may be needed—as in the case of fixed lights placed at the end of narrow channels, and of revolving lights which are made visible all round by causing the lenses and reflectors to revolve about the source of light, or with that source about a centre.[17] [17] _Edinburgh Review_, Jan. 1862, pp. 178, 179. Two methods have been employed for the purpose of throwing light in the desired direction: first, by silvered parabolic reflectors, which is called the _Catoptric System_; second, by the employment of lenses of a peculiar construction, which is known as the _Dioptric_ (or _Refracting_) _System_. Occasionally these two systems are combined, as in the ordinary _Catadioptric_, and in Mr. Stevenson’s admirable _Holophotal_ arrangement, whether Catoptric or Dioptric. Before describing them, however, it will be desirable to offer a brief history of lighthouse illumination. It was at a comparatively recent epoch that wood and coal fires were for the first time replaced by candles, and the open summit of the tower covered in with glass. About the end of the eighteenth century, for these insufficient producers of light, lamps were substituted, whose lustre was directed to a distance by reflectors of polished metal. Many of the lighthouses of this epoch were provided with the species of apparatus here described; among others, those of Capes de l’Ailly and de la Hève, the isles Rhé and Oléron. In 1782, an identical mode of lightage was established at Cordouan; but though this lighthouse did not include less than twenty-four lamps, accompanied each by a reflector, it diffused so feeble a light, that the seamen immediately insisted on a return to the barbarous system of the Middle Ages.