Another and more important precaution consists in keeping always at hand, in the light-room, a _spare lamp_, trimmed, and adjusted to the proper height for the focus, and in every respect ready to act as a substitute for the other if any accident occurs. But while I am tracing these words, I read that experiments have been successfully made with _gas_ for the illumination of the lenticular apparatus, and that, if it will afford a steadier and fuller light, at less expense, and with no risk of accident, it will probably be adopted. * * * * * To continue:— Once having acquired a full command of all the rays amplified from the lamp, the next desideratum was to diversify the appearance of the light which they constituted; for, as I have already said, it is not enough to stretch a belt of warning fires around the coast,—we must take care that each shall in some wise be distinguished from the other, so as to afford the navigator a clue to its particular locality. Hence arose the division into fixed, revolving, intermittent lights, and so on, which I have already described, and which is secured in the following manner:— [Illustration: FRESNEL’S REVOLVING LIGHT.] 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. [Illustration: STEVENSON’S HOLOPHOTAL REVOLVING LIGHT.]