In France there is frequently employed what Fresnel called a “fixed light varied by flashes.” This effect is produced, as already explained, by causing panels of glass, curved horizontally but not vertically, to revolve outside of Fresnel’s fixed apparatus, as shown in the diagram. The ordinary fixed apparatus only acts in the vertical plane, while the straight panels only act in the horizontal plane. So that when the fixed apparatus is alone visible the rays are only gathered from the vertical plane, and the light is comparatively weak; but when the panels come opposite the eye, the rays are gathered from both planes into one powerful beam, as in a revolving light. Here, as in the former case, two agents are employed, causing great loss of light and great unnecessary expense, where one, if of the proper form, is sufficient. Mr. Stevenson’s modification of the holophotal arrangement for this purpose is shown in the diagram. It consists of alternate panels of the fixed light and holophotal apparatus; and thus, by single agency, a weak fixed light and a stronger revolving light are shown time-about to the mariner, which is the required characteristic.
FRESNEL’S FIXED LIGHT APPARATUS.
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.
