Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/642

 620 LIGHTHOUSE Central lamp. Cata- dioptric fixed light. Dioptric fixed light. Section Elevation .

along AB, and after a second refraction at B emerge horizontally. The lowest ray FB after refraction by AB must, for like reason, pass along BC, and after reflexion by AC and a second refraction by BC also emerge horizontally. Every other ray incident on the prisms between A and B is, after one reflexion and two re fractions, emitted horizontally. Straight Refracting Prism. Fresnel s straight refracting prism which refracts the rays that fall on it, but in one plane only, requires no further explanation, as it is simply a straight prism of the same horizontal cross section as one of the prisms of his cylindric refractor, so as when placed in front of his fixed ap paratus to produce a beam of parallel rays like a lens. Great Central Lamp. &quot;We will now goon to describe the manner in which Fresnel utilized the four new optical agents which he originated, by first referring to his central burner system. In all lighthouses prior to 1822 the mode of getting up the required power was by employing a sufficient number of separate reflectors, each of which (unless we except Bordier Marcet s mirror) required its own separate lamp. Instead of numerous independent lamps and reflectors, Fresnel used a single lamp which had four concentric wicks, and was fed with oil by a pump worked by clock work. Surrounding this burner was a stationary cylindric refractor for a fixed light, and annular lenses -evolving outside of it for a Devolv ing light. FrcsncTs Combinations of his Optical Agents. Catadioptric Fixed Light. This apparatus (figs. 17 and 18), in which a central burner is used, consists of a dioptric cylindric refractor with zones of silvered mirror above and below similar in profile to Bordier Marcet s reflector. By the adoption of the FIG. 17. Section. Fia. 18. Plan. refractor the whole of the wasteful divergence which occurs in Marcet s reflector is prevented. We have here a geometrically perfect combination, but it is not so physically, because metallic reflexion is used. This physical defect Fresnel obviated in his next design. Dioptric Fixed Light. First Application of Total Reflexion to Fixed Lights. In this apparatus Fresnel substituted his totally FIG. 19. Elevation. FIG. 20. Vertical Section. reflecting prism p and lens R for Marcet s reflector, and thus distributed the whole light equally over the horizon by means of dioptric agents alone. This was the first application of total reflexion to lighthouse apparatus, and this beautiful instrument continues till now in universal use. Figs 19 and 20 represent an elevation and section of this apparatus. FresncTs Revolving Light. In this form of revolving light (fig. 21) the central burner is surrounded by annular lenses L, and a compound arrangement of inclined trapezoidal S &amp;gt;X.M lenses L and plane silvered mirrors M. The inclined lenses fit closely to each other and form a pyra midal dome, and the light, intercepted by them, is sent upwards in inclined beams until, falling upon the plane mirrors M, it is sent outwards in horizontal FlG - 21. Vertical Section, parallelized beams. All these optical agents are made to revolve round the central lamp, and the sailor receives a full flash when the axis of one of the emerging beams passes his eye, and as soon as it passes him he is in dark ness until the next beam conies round. This de sign, unlike that of his fixed light, is imperfect on account of the employ ment of metallic reflexion, and because two agents arc employedfor all except the central portion of the rays. Fixed Light varied by Flaslics. This distinction (figs. 22 and 23) Fresnel produced by placing his straight refracting prisms / on a revolving frame outside of his fixed light apparatus rr, so that when the upright prisms come in line with the observer the light is in creased to the power of the revolving light, a broad flash as in the annular lens bein Revolv ing light. FIG. 22. Plan. Fixed light with flashes. Fia. 23. Vertical Section. produced in place of a narrow strip of rays as in the unassisted fixed light. Alan Stevenson s Improvements. Mr Stevenson was the first to introduce the dioptric system into Alan Britain, and in doing so he made the following improvements. Steven- 1. Refractor of a Truly Cylindrical Form. Owing to difficulties son s im in construction, Fresnel adopted a polygonal instead of a cylindric prove- form for his refractor, but Mr Stevenson succeeded in getting Messrs ments. Cookson of Newcastle to construct a first order refractor of a truly cylindric form. 2. Helical Glass Joints for Fixed Lights. Mr Stevenson further improved the apparatus by constructing the refractor in rhomboidal instead of rectangular pieces (figs. 24 and 25), thus producing helical joints and preventing serious obscuration of the light in any azimuth. 3. Helical Metallic Framings. The internal metallic frame work for supporting the upper cupola of prisms was also, for the same reason, made by him of a helical form. 4. Diagonal Framed Lantern. The astragals or sash bars of the lantern were likewise made dia- d FIG. 24. Ele vation. Frc. 25. Vertical Section. gonal and constructed of bronze instead of iron in order to reduce their sectional area. A. small harbour light with inclined astragals was made in 1836 by Mr E. Sang. Mr Stevenson also prepared a design in 1846 for Start Point, Orkney, in which he extended the helical arrangement to the astragals, but it was never carried out. Mr Douglass independently designed and afterwards carried into practice this form of lantern. Alan Stevenson s Improved Revolving Light for SJccrryvorc. In 1835 Mr Stevenson, in a report to the Northern Lighthouse Board, proposed to add fixed reflecting prisms p (fig. 26) below the lenses of Fresnel s revolving light, and he communicated this proposal to M. L. Fresnel, who approved of his suggestion, and