[Illustration: FRESNEL’S FIXED LIGHT APPARATUS.]
[Illustration: FRENCH FORM OF APPARATUS FOR A FIXED LIGHT VARIED BY FLASHES.]
If, for the sake of further diversity, the lights are to be coloured, we content ourselves in the case of a fixed light, as already stated, with enclosing the flame in a green or red tube. In the French eclipsing apparatus, polished sheets of coloured glass are placed, on one side or the other, against the lenses intended to emit the flashes of colour.
While doing all honour to Fresnel and his great invention, we must not forget that its present comparative perfection is due to Mr. Thomas Stevenson, whose improvements, in truth, have eventuated in almost a new system, now known as the _Holophotal_,[22] and already partially described. By a peculiar combination of dioptric spherical mirrors and other apparatus, it also succeeds in economizing and condensing into one beam the whole of the rays thrown off from the burners; but I fear that any explanation of it which could here be attempted would, from its necessary introduction of technical language, prove unintelligible to the non-scientific reader.
After having devoted so many pages to what we have called—perhaps somewhat fancifully—the _soul_ of the lighthouse, it becomes necessary to say a few words in reference to the _body_ which encases it.
In building up this body, no less ingenuity and science have been displayed than in perfecting and expanding the light which gives it life and value. Whether the lighthouse-tower is situated on some wave-washed rock surrounded by a hungry sea, or on the summit of a conspicuous headland, the highest skill is exercised upon its construction, and it becomes, in many instances, a monument of the most brilliant architectural genius. Not, indeed, that it exhibits those beautiful features of clustered columns and lofty arches, or that elaboration of picturesque ornament, which delight us in the lordly mansion and the ancient cathedral; but that an equal perfection of art is revealed in its massive simplicity and impregnable solidity, and in its admirable adaptation to the grand purposes for which it is intended.
Two primary conditions, it is obvious, must always govern the construction of a lighthouse: it must be raised to an elevation suitable for the full display of its warning radiance, and it must be built with a strength and solidity which will defy the assault of wind or wave.