If a fixed light be required, the apparatus as invented by Fresnel takes the form of an annular glass frame produced by the revolution of the section passing through the centre of a circular lens, and reflecting prisms around a vertical elevated on the principal axis of this section, as shown in the diagram.
For revolving apparatus, Fresnel’s apparatus, as employed in all lighthouses prior to the introduction of the holophotal arrangement, consisted, as may be seen in the diagram, of annular lenses, L, for acting on the central part of the light, while the upper rays were refracted by inclined hanging lenses, a, and ultimately reflected into the proper direction by silvered mirrors marked b, placed above. The lower rays were intercepted by fixed light prisms, p p (which did not revolve), and which, showing a fixed light all round, were, of course, of very inferior power to the solid beams proceeding from the large lenses L, and the smaller lenses and mirrors placed above. Strictly speaking, Fresnel’s revolving light consisted of a revolving and a weak fixed light. As the frame revolved round the central lamp, the mariner saw the luminous beam when the lenses were turned towards him, and the number of flashes depended on the quickness of the rotation.
STEVENSON’S HOLOPHOTAL REVOLVING LIGHT.
The apparatus adopted by Fresnel for the fixed light may be regarded as perfect; but his revolving light has been now superseded by the holophotal apparatus of Mr. Thomas Stevenson. The inclined mirrors and lenses employed in Fresnel’s apparatus are done away with in Mr. Stevenson’s, shown in the diagram, in which, by the single agency of lenses, L, and totally reflecting prisms, p, all the rays are rendered parallel. In this form the whole glass frame, consisting of lenses and reflecting prisms, revolves round the central lamp. As Fresnel’s lighthouse prisms only gathered the light vertically, they could not produce the sheaf of rays required for the revolving light unless when combined with others which gathered the rays horizontally. The first lighthouse in which single prisms were made to revolve was the Horsburgh light, near Singapore, the apparatus of which was designed by Mr. Stevenson in 1850. In this form of revolving light apparatus the prisms are generated about a horizontal instead of a vertical axis, as in fixed light. The forms of the beams of light issuing forth from Fresnel and Stevenson’s apparatus are shown opposite to the diagram of each, and marked x, y.
STEVENSON’S FIXED LIGHT VARIED BY FLASHES.