Lighthouse Structures

Fresnel also designed a form of fixed and flashing light in which a fixed light, varied by flashes, was produced by placing panels of straight refracting prisms in a vertical position on a revolving carriage outside the fixed light apparatus. The revolution of the upright prisms periodically in creased the power of the beam by condensation of the rays, emergent from the fixed appara tus, in the horizontal plane.

The lens segments in Fresnel's early apparatus were of polyg onal form instead of cylindrical, but subsequently manufacturers succeeded in grinding glass in cylindrical rings of the form now used. The first apparatus of this description was made by the Cooksons of Newcastle in 1836 at the suggestion of Alan Stevenson, and erected at Inchkeith, Fifeshire. The first dioptric apparatus erected by the Trinity House to show a fixed light was the one formerly at Start Point in Devonshire. It was con structed in 1836.

Azimuthal Condensing Prisms.—To condense the rays from a fixed-light apparatus in certain azimuths, T. Stevenson devised in 185o his azimuthal condensing prisms which have been vari ously applied in the construction of optical apparatus as, for instance, for the strengthening of colowsed sectors. Applications of this system will be referred to subsequently (see fig. 1o).

Dioptric Mirror.—An important improvement in lighthouse optical work was the invention of the dioptric mirror by (Sir) J. T. Chance in 1862. This mirror is a modification of a spheri cal mirror devised by T. Stevenson in 185o, in which double re flection from the internal surfaces of a catadioptric prism was employed. Chance generated the zones of prisms round the vertical axis, separated the elements and set them at increasing radii, thus producing an instrument of practical utility. By the use of the dioptric mirror rays of light which would other wise be lost are reflected back through the focus of the source of light and are refracted or reflected with the main rays. This form of mirror is still in general use and is constructed for verti cal angles up to t00°.

Spherical Lens.—Mr. C. A. Stevenson devised in 1888 annular lens panels consisting of lens elements spherical in the horizontal and vertical planes, and these, with equiangular prisms, have been used in a number of apparatus for Scottish lighthouses.

Optical Glass for Lighthouses.

In the early days of lens lights the only glass used for the prisms was made in France at the St. Gobain and Premontre works, which have long been celebrated for the high quality of optical glass they produce. The early dioptric lights erected in the United Kingdom, some 13 in all, were made by the Cooksons, who were instructed by Leonor Fresnel, the brother of Augustin. At first they tried to mould the lens and then to grind it out of one thick sheet of glass. The manufacture of lenses was abandoned by Cooksons' successors in 1845 and in 185o Chance Bros. and Co., of Bir mingham began to make optical glass, assisted by M. Tabouret, a French expert who had been a colleague of Augustin Fresnel himself. The first light made by the firm was shown at the Great Exhibition of 1851, since when numerous dioptric apparatus have been constructed by Messrs. Chance, who are, at this time, the only manufacturers of lighthouse glass in the United Kingdom. Most of the glass used for apparatus constructed in France is manufactured at St. Gobain. Some glass used by German con structors is made at Rathenau in Prussia and Goslar in the Harz.

The glass generally employed for lighthouse optics has a mean refractive index of p. = 1.51, the corresponding critical angle be ing 41° 3o'. Messrs. Chance have used dense flint glass for the upper and lower refracting rings of high-angle lenses (up to 97° vertical angle) and for dioptric mirrors in certain cases. This glass has a value of ,u =1.62 with critical angle 38° 5'. The use of refracting elements for an angle greater than 6o° aperture is not attended by any advantage over reflecting prisms.'