have any record is that which was erected on the island of Pharos, by Ptolemy Philadelphus, in the year 470 of the foundation of Rome. This was merely a tower, upon the top of which fires were kept burning at night; but as the world progressed, the blazing tar-barrel or wood fire gave place to the carefully-constructed lamp and silvered reflector apparatus, which are fast disappearing in their turn before the electric or Drummond light and the refracting apparatus constructed by Fresnel, who was the first to endeavour to abolish the old-fashioned and inefficient metallic mirror from the lanterns of lighthouses. Fig. 35 shows a section of Fresnel's apparatus. A is a plano-convex lens of about a foot in diameter, whose focus corresponds with those of the concentric lenticular rings of glass which surround it, and which are seen more plainly in fig. 36. These rings, which are ground and polished with the greatest accuracy, are somewhat in the shape of an ordinary quoit, and are equivalent to a plano-convex lens with the centre portion cut out. This arrangement is so powerful that the distance at which a light provided with it can be seen is only limited by bad weather, the state of the atmosphere and the distance of the horizon. It is common for such lights to be seen at a distance of between fifty and sixty miles. The apparatus is mostly arranged in the form of an octagon, and is generally provided with additional reflecting mirrors at those parts above the light which are out of the range of the lenses. The light shining fully in eight directions at one time, can scarcely be missed by any ship within range; but in order to guard against any possibility of accident, the optical apparatus is often made to revolve by clockwork, so that every point of the ocean is illuminated in turn. By using coloured glasses, or by causing the light to disappear at distinct intervals, different lighthouses may be identified by
