Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/641

 LIGHTHOUSE 619 glass, and made, as he says as nearly as they can be to the parabolic curve. &quot; This is unquestionably the earliest published notice of the use of parabolic reflectors for lighthouse illumination. Up to 1782 the wicks of the lamps were of a flat form, but in that year Argaud introduced wicks and burners of a hollow cyliudric form which admitted a central current of air through the burner so as to ignite the cone of gas issuing from the wick both within and without. Rumford afterwards split up the cone of gas into several concentric shells. &quot;It is remarkable,&quot; says Mr J. T. Chance in his excellent memoir (Min. Ins. Civil Eng., vol. xxvi.), &quot;how many inventors have contributed their respective parts to the multiple burner : Argand, the double current; Lauge, the indispensable contraction of the glass chimney; Carcel, the mechanism for an abundant supply of oil; and Count Rumford, the multiple burner, an idea made feasible by these contrivances, and finally realized by Arago and Augustin Fresnel. &quot; Optical Properties of the Parabolic Reflector. In the parabolic reflector all rays diverging strictly from the focus and falling on the paraboloid emerge in one beam of parallel rays. But as an oil light is not a mathematical point, but an object of considerable magni tude, the rays from the outside of the flame being exfocal will, after reflexion, emerge as a cone whose divergence is directly pro portional to the radius of the flame and inversely to the focal dis tance of the reflector. Its intensity must consequently vary as the squares of the dfstances from the lighthouse. Optical apparatus does not then prevent that divergence which is due to the flame being of sensible magnitude. Defects of the Paraboloid. It will be seen from fig. 7 that the parabolic mirror a is at best but a very imperfect instrument, for even if the radiant were strictly a mathematical point, the cone of rays (shown undotted) escaping past the lips of the mirror must be lost. Vertical Section. FIG. 8. Plan. Arrange- Mode of Employing Reflectors for Fixed and Revolving Lights. In meut of order to produce, on the catoptric system, a fixed light showing all refleo round the circle, a number of reflectors (o, o, o, fig. 8) are fixed tori. round the outside of a stationary chande lier n. As the ordinary paraboloid has about 14 of divergence, twenty-five re flectors were needed to light up continu ously (though not equally) the whole horizon. If again the light was to revolve, then a revolving chandelier (figs. 9 and 10) was employed having a certain number of flat faces, on each of which was fixed a number of separate lamps and reflectors with their axes parallel to each other. When the chandelier revolved, and one of the flat sides was turned towards the sailor, hi would, when at some distance from the shore, receive a flash at once from each of the mirrors which were on that face, but when the face was turned away from him a dark period would intervene until the next face came round again. Fanal Hardier Marcet s Fanal Sideral, 1819. In siik ral. order strictly to equalize a fixed light over the whole horizon, which could not possibly be done with separate reflectors, Marcet proposed this ingenious instrument, which is generated by the revolution of the para bolic profile pp (fig. 11) round its para meter as a vertical axis, instead of round a horizontal axis, as in all former reflectors. The vertices of the parabola are cut off, so as to permit of a common focus for the ilame. The rays will therefore be reflected FIG. 10. Plan. parallel to the horizontal axis in the vertical plane, while the natural divergence of the light in azimuth will not be interfered with. By this excellent contrivance the light was for the first time spread equally round the horizon in one continuous zone. But even though the radiants were reduced to a mathematical point, very many of the rays (shown in hard lines in the elevation, fig. 11) are allowed to escape past the lips of the reflector, and this loss takes place all round the circle. DIOPTRIC SYSTEM. Beginning in 1822, Augustin Fresnel, the eminent physicist and Dioptric mathematician, entirely revolutionized the previously existing light- system. house system by means of his annular lenses, cylindric refractors, and totally reflecting prisms. Before describing these and their combinations it is necessary to state that the size of the flame pro duces divergence with lenses as well as with reflectors. The measure of this divergence for any point of the lens is the angle whose sine is Radius of flame Distance of point from centre of flame FresneTs Optical Agents, Annular Lens, 1748-1822. Buffon in 1748 suggested a new form Fresnel s of lens for burning purposes in order to save the loss of heat by annular absorption of the sun s rays in passing through a thick lens of large lens. size whose outer profile is continuously spherical. He proposed to grind out of a solid piece of glass a lens in steps or concentric zones in order to reduce the thickness to a minimum (figs. 12 and 13). Condorcet, in his logc de Buffon, in 1773 (Paris edition, 1804, p. 35) pro posed the capital improvement of building ur&amp;gt; Buff on s stepped lens in separate rings, and pointed out that the cutting of the surface into steps had the effect of correcting to a large extent the spherical aberration, or divergence from the parallel, of the rays emitted by any continuously G- splierical lens. Sir D. Brewster, in 1811, also described the same plan. But both these writers designed their lenses for burning purposes only, and not for operating on light, while all the surfaces of their lenses were spherical. In 1822 Fresnel constructed a built-up lens for lighthouse purposes, in which the centres of curvature of the different rings receded from the axis according to their distance from the centre, so as prac tically to eliminate spherical aberration, the only spherical surface left being the small central part a (fig. 13). These lenses are used for revolving lights only. Cylindric Refractor. This instrument was introduced by Fresnel Cylindric for effecting dioptrically by refraction in front of the flame what refractor. had been done before catop- trically by Marcet s reflector by reflexion from behind the flame. It consisted of a zone or hoop of glass (figs. 14 and 15) generated by the revolution round a vertical axis of the middle section of the annular lens just de scribed, which lens, on the other hand, being generated by the same profile round a horizontal axis, parallelized the rays in every plane, whereas the cylindric re- _, _ pl fractor does so in the vertical plane only. ^ Totally-Reflecting Prisms. _ 1 ^*_ --- 1 Prisms. Fresnel next conceived the admirable improvement of employing the principle of &quot;total&quot; or internal re flexion by glass prisms. The ray Fi (fig. 16) falling on a prismoidal ring, ABC, is refracted and bent in the direction? R, and falling on the side AC, at an angle of incidence greater than the critical, is totally reflected in the direction EC, and, impinging on the side BC at c, it undergoes a second refraction, and emerges horizontally. The highest ray FA after refraction by AB and reflexion by AC must (in order to avoid superfluous glass) pasa FIG. 15. Vertical Section.