The application to this purpose of deep metallic mirrors, known under the name of parabolic mirrors, has been found effectual to the purposes required. When a lamp is placed in the focus of such a mirror, all the rays which emanate from it are reflected from the polished surface, and converge in one direction: their original divergence is destroyed, and they form, as they issue from the apparatus, a cylinder of light, parallel with the axis of the mirror. This light would be transmitted with undiminished brilliancy to a great distance, did not the atmosphere absorb a portion of it. It must, however, be admitted, that this method is not free from defect. It is true, we direct towards the horizon of the sea, a vast number of rays, which would have been lost upon the ground, in space, or landward; we also destroy the primitive divergence of those rays which fall within the range of the seaman; but the cylinder of reflected light is of no greater size than that of the mirror; the zone which it illuminates has precisely the same dimensions, at whatever distance, and, unless we employ a number of similar reflectors differently inclined, there will be a number of large spaces in the horizon completely obscure, from which the pilot will never see any signal whatever. This serious objection has been removed by imparting, by means of clock-work, a uniform rotatory motion to the reflector. The collection of rays proceeding from the mirror is thus directed to all the points of the horizon in succession. Every vessel perceives the signal-light during one instant, and immediately after it is seen to disappear; and if, in a great extent of coast, the different lights revolve in different times, the various signals become thus individualized. According to the interval of time, which elapses between two successive appearances or eclipses of the light, does the sailor recognize the part of the coast which is in view: he is thus no longer liable to mistake a planet, or a star of the first magnitude, at its rising or setting, or a fire lighted on the coast by fishermen, charcoal-burners, &c. for the light of the lighthouse; mistakes, which have often led to the most deplorable wrecks. The reflectors originally employed were casts in plaster of Paris, from a mould formed to the parabolic curve, and lined with facets of mirror-glass. The power of these reflectors, however, was comparatively small, from the reflecting surface being composed of numerous pieces, in each of which only one point coincided with the curve of the parabola.