of a zone of glass generated by the revolution round a vertical axis of a medial section of the annular lens (fig. 33). The lens belt condensed and parallelized the light rays in the vertical plane only, while the annular lens does so in every plane. The first revolving light constructed from Fresnel's designs was erected at the Cordouan lighthouse in 1823. It consisted of 8 panels of annular lenses placed round the lamp at a focal distance of 920 mm. To utilize the light, Dhu Heart, ls-t Course.

I ' " Ig | I l Q g4¥'fm U"€§ |=|I||F!' l@§ 'w Pv | | |55 *Q- af Bell Bock FIGHT. X T" - v "il fi ' sw QQ" fn?

1/ S . t T er. Wolf, 12th Course. mea on S ow

M 50 *Q 18 ' QW al lt! $4 *, E“'*s;i'.;f.;.';*'¥;, ';2P'“ cn ken me. 7' . I ' c scuczaof »-fr C'0'%N5l'9, J? I M gt -, , '- e-FIG. 22.-Courses of various Lighthouse Towers. which would otherwise escape above the lenses, Fresnel introduced a series of 8 plain silvered mirrors, on which the light was thrown by a system of lenses. At a subsequent period mirrors were also placed in the lower part of the optic. The apparatus was revolved by clockwork. This optic embodied the first combination of dioptric and catoptric elements in one design (fig. 34). In the following year Fresnel designed a dioptric lens with catoptric mirrors for fixed light, which was the first of its kind installed in a lighthouse. It was erected at the Chassiron lighthouse in 1827 (fig. 35). This combination is geometrically perfect, but not so practically on account of the great »-"“"" r" Y . FIG. 23.-Perspective drawing of Dovetailed FIG. 24.-Section Stone (Vl/olf Rock). of Dovetail. loss of light entailed by metallic reflection which is at least 25% greater than the system described under. Before his death in 1827 Fresnel devised his totally reflecting or catadioptric prisms to take the place of the silvered reflectors previously used above and below the lens elements (fig. 28). The ray is refracted in the direction 1$ r of incidence greater than the 5 critical, is totally reflected in Fi falling on the prismoidal ring ABC and meeting the face AB at an angle - - - #Mr the direction r e emerging after second refraction in a -'°A£°£°LP£"”"* j'~" horizontal direction. Fresnel I, , i§ , ,, devised these prisms for use 1 *Mm* in fixed light apparatus, but l uggizn the principle was, at a later { 1 f<;”WJ5&l'. | f f!5§ l» 5. = llflllll ¢, ¢..l@., , Q 1 w | J", Qs ¢ »uum', fill ' " " I I, / !§ 'hl Iilllllll; !|IIEll 5 ¢¢$$~s颧 5 mi ni = W%Wy¢

| u|nun|nm~ . l u I ar '<'=f';a '~':, i.—§ E¥;»ii:E-i?t1r =H~7" L -=¢t';1:;%':¢I» —'— ., ,, ,.. f . -'v::~ 4;-:.;':;' 11:f V 9" " '""~' ' " ' ' JL ef-T" ' ers-' an £1 FIG.25.—Dassen Island FIG. 26.-Cape San Thomé Lighthouse (cast iron). Lighthouse. date, also applied to flashing lights, in the first instance by T. Stevenson. Both the dioptric lens and catadioptric prism invented by Fresnel are still in general use, the mathematical calculations of the great French designer still forming the basis upon which lighthouse opticians work. Fresnel also designed a form of fixed and flashing light in which the distinction of a fixed light, varied by flashes, was produced by placing panels of straight a revolving carriage outside the fixed light apparatus. The revolution of the upright prisms periodically increased the power of the beam, by condensation of the ra s emergent from the fixed apparatus, in the horizontal plane. The lens segments in Fresnel's early appara G refracting prisms in a vertical position on W % ... - /, f . .:- / / A /'¢, "' ¢" a 1 ¢¢7 na » f FIG. 27.-Dioptric Prism. tus were of polygonal 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 Messrs Cookson of Newcastle in 1836 at the suggestion of Alan Stevenson and erected at Inchkeith. In 1825 the French Commission des Phares decided upon the exclusive use of lenticular apparatus in its service. B r The Scottish Lighthouse "' """ 'Z' A Board followed with the C, ' Inchkeith revolving ap-paratus in 1835 and the Isle of May fixed optic in

1836. In the latter instru- "ment Alan Stevenson in- ° troduced helical frames for holding the glass prisms in

place, thus avoiding complete obstruction of the ' light rays in any azimuth. The first dioptric light erected by the Trinity House was that formerly at Start Point in Devonshire, constructed in 1836. Catadio tric or reflecting prisms fldr revolving lights were not used until 1850, when Alan Stevenson designed them for the North Ronaldshay lighthouse.

Fefe § ~ Pa=@ . F FIG. 28.-Catadioptric or Reflecting

Prism.