Lastly, to test the true position of the lamp itself, with reference to the lenses thus properly arranged, we apply a radius or trainer ([fig. 90]) which fits into the centre burner at F, while its point A touches the centre of the lens l, l; at B is a graduated slide, which admits of the trainer being lengthened or shortened to suit various focal distances; and the spirit-level at c at once corrects any error in the length of the trainer arising from depression or elevation, and also serves to indicate the proper level for the burner which is noticed at [page 290], in speaking of the lamp. The dotted line F c′ B′A′ shews the position of the trainer in reference to an adjacent lens.
The elegant apparatus invented by Augustin Fresnel for Harbour Lights, on the same principle as that just described for Sea Lights, is shewn beneath ([fig. 91]). It consists of thirteen rings of glass of various diameters arranged one above another, in an oval form. The five middle rings have an interior diameter of 11·81 inches (30cm.), and like those of the larger apparatus, refract equally over the horizontal plane of the focus the light which they receive from it. The other rings or prisms, five of which are upper and three lower, are ground and set in such a manner, that they project all the light derived from the focus in a direction parallel to the other rays by total reflection.
Fig. 91.
The arrangements of this apparatus, which is distinguished by the addition of external refractors arranged vertically, will be more fully understood by a reference to [fig. 91], which shews its section and plan. F is the focal point in which the flame is placed; r, r cylindric refractors, forming by their union a cylinder with a lamp in its axis, and producing a zone of light of equal intensity all round the horizon; x, x are catadioptric prismatic rings acting by total reflection, and giving out zones of light of equal intensity at every point of the horizon. The dotted lines shew the course traversed by the rays of light which proceed from the lamp, and are acted upon by the rings of glass. The letters r′ r′ shew the external prisms, having their axes at right angles to those of the principal bent prisms, composing the refractors at r, r, and revolving around them. This ingenious application of the property of crossed prism is already described at [page 264].
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:—
| Number of Zone. | Radii of curvature. | Horizontal distance from the axis of the system. | Vertical distance from the outer arris. | Number of Zone. | Radii of curvature. | Horizontal distance from the axis of the system. | Vertical distance from the outer 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. | 1250·0 | 150·67 | 1211·90 | V. | 1250·00 | 452·23 | 1184·20 |
| II. | 1250·0 | 228·19 | 1208·80 | VI. | 1250·00 | 453·07 | 1185·20 |
| III. | 1250·0 | 305·10 | 1203·35 | VII. | 1250·00 | 374·60 | 1196·00 |
| IV. | 1250·0 | 381·00 | 1195·30 | VIII. | 1250·00 | 294·35 | 1203·80 |
Power of Dioptric Instruments. The effect of an annular lens, in combination with the great lamp, may be estimated at moderate distances to be nearly equal to that of 3000 Argand flames of about an inch diameter; that of a cylindric refractor at about 250; and that of a curved mirror may perhaps on an average be assumed at about 10 Argand flames.