When this apparatus is employed to light only a part of the horizon, the rings are discontinued on the side next the land, and room is thus obtained for using a common fountain lamp; but when the whole horizon is illuminated, the apparatus must inclose the flame on every side; and it has in that case been found most convenient to employ the hydrostatic lamp of THILORIER, which has a balance of _sulphate of zinc in solution_. An instrument differing from this small apparatus only in size, has lately been introduced into the Lighthouses in France, and has also been adopted in Scotland for lights in narrow seas. It has the same number of rings of glass as the small apparatus, and of the same _proportional_ dimensions. Its internal diameter, however, is 500 millimètres (about 19¹⁄₂ inches). Drawings of the smaller apparatus are given at Plate XIX., which also contains the radii and the centre of curvature for the rings of the central dioptric belt; while the following Table gives the elements of the eight prismatic zones (above and below the belt), with the co-ordinates to their centres of curvature, measured from the arris A of the outer or emergent surfaces, in whatever position the zone may lie on the lathe. The dimensions are in millimètres; but may be easily converted into imperial inches, in the manner described in the Table of the Great Zones, which will be found in the Appendix:-- +------+-------+--------+--------++------+-------+--------+--------+ | | | Hori- | || | | Hori- | | | | | zontal | || | | zontal | | | | |distance|Vertical|| | |distance|Vertical| | | Radii |from the|distance|| | Radii |from the|distance| |Number| of | axis of|from the||Number| of | axis of|from the| | of | curva-| the | outer || of | curva-| the | outer | | Zone.| ture. | system.| arris. || Zone.| ture. | system.| arris. | +------+-------+--------+--------++------+-------+--------+--------+ | REFLECTING SURFACES (CONVEX). | | I. | 1094·5| 723·5 | 833·78|| V.|1222·9 | 523·6 | 1169·1 | | II. | 1045·7| 652·8 | 891·01|| VI.|1249·4 | 544·0 | 1192·1 | | III. | 1044·5| 598·4 | 933·67|| VII.|1150·4 | 593·5 | 1062·9 | | IV. | 1087·3| 551·9 | 1010·20|| VIII.|1113·6 | 650·5 | 985·9 | +------+-------+--------+--------++------+-------+--------+--------+ | OUTER REFRACTING SURFACES (CONCAVE). | | I. | 1250·0| 1290·7 | 303·19|| V.|1250·00| 1100·22| 829·52| | II. | 1250·0| 1274·2 | 445·89|| VI.|1250·00| 1112·74| 821·16| | III. | 1250·0| 1234·0 | 587·05|| VII.|1250·00| 1190·50| 698·00| | IV. | 1250·0| 1173·0 | 717·71|| VIII.|1250·00| 1251·90| 557·22| +------+-------+--------+--------++------+-------+--------+--------+ | INNER REFRACTING SURFACES (CONVEX). | | I.