Fig. 305.—Revolving Light Apparatus.
Fresnel saw that it would be useless to apply lenses in lighthouse illumination unless the intensity of the light given out by the single-wick Argand lamps then in use could be considerably increased, without much enlarging the flame. Accordingly he devoted himself, in conjunction with his friend Arago, to this preliminary consideration. Their studies and experiments led them to the construction of the lamp with several concentric wicks—by which a brilliancy of light is obtainable twenty-five times greater than that of the single-wick Argand. The light which the improved lamp, when combined with Fresnel’s lenses, could send to the horizon, was equivalent to that which would be given by the united beams of 4,000 Argand lamps without optical apparatus; and it was eight times greater than any which could be produced by the reflectors then in use. The first apparatus constructed on Fresnel’s plan was placed on the Tour de Cordouan in July, 1823.
France led the van in the erection of the most perfect lighthouses in the world, and it was not until 1835 that, by the strenuous advocacy of Mr. Alan Stevenson, a dioptric apparatus was employed in a British lighthouse; but at the present time Fresnel’s principle has been adopted in the majority of British lighthouses. Fig. [305] is a part elevation, with the section, of a catadioptric apparatus of the first class. In plan it is a regular octagon, and it sends out eight beams, which are directed to the horizon, and made to sweep over the sea by its regular rotation, produced by clockwork contained in the case, A. The whole frame is very accurately balanced, and turns on its bearings, and the rollers, h, h, with great smoothness and steadiness. The moving power is given by the descent of a weight attached to a chain or cord, which is wound round a barrel. One train of wheels is connected with apparatus for regulating the speed, and to this an indicator is attached which registers the number of revolutions made in an hour. There is also a contrivance of some kind for maintaining the motion while the weight is being wound up. The reader will observe that all the light of the lamp, L, is utilized, except that which is directed towards the base and top of the apparatus—a quantity less than one-fifth of the whole. About 45 per cent. of the light emitted by the lamp falls on the refracting lenses; 22½ on the upper reflecting prisms; and 13½ on the lower reflecting prisms. The brightest part of the flame is placed so that the beams from it are directed towards the sea horizon, and the space between the horizon and the neighbourhood of the lighthouse receives ample light from the other parts of the flame. Thus a ship, or any part of the sea within the range of the lighthouse, will see the light appearing at regular intervals, as one after another of the eight beams passes across it, the intervals being one eighth of the time in which the apparatus completes its revolution. The zones of totally reflecting prisms, shown at e e´, f f´, Fig. [305], were not adopted in British lighthouses until 1844, when the Skerryvore light was exhibited with the complete apparatus represented in the drawing.
The optical apparatus for lighthouses is constructed of certain sizes, adapted to the different situations in which it is to be used. The apparatus we have just described is made in six forms, according to the order of light required. The first three orders are for sea lights, the rest for harbour lights; and the following are the dimensions of the apparatus for each order of revolving or fixed lights:
| Order. | Height in Inches. | Internal Diameter in Inches. | Number of Reflecting Prisms. | |
|---|---|---|---|---|
| In Upper Zone. | In Lower Zone. | |||
| 1 | 106½ | 72½ | 18 | 8 |
| 2 | 83½ | 55 | 16 | 4 |
| 3 | 61½ | 39½ | 11 | 4 |
| 4 | 29 | 19¾ | 5 | 4 |
| 5 | 21¾ | 14¾ | 5 | 4 |
| 6 | 17½ | 12 | 5 | 4 |
When a revolving apparatus of the above description is erected on shore, a reflector of suitable shape and dimensions is placed on the landward side of the lamp, so as to throw its rays back upon itself and towards the lenses which are directed seaward.
Fresnel also constructed glass apparatus for fixed lights. If we require to send the light equally towards the horizon in all directions at once, the problem is capable of solution, either by a proper form of glass apparatus or by a proper form of mirrors. Suppose the section, e c f, Fig. [305], to revolve about a vertical axis passing through the lamp, it would sweep out a form which, when executed in glass, would spread out all the light falling upon it into one horizontal sheet. Fresnel was obliged to content himself with an approximation to this shape, formed by a prismatic frame of many sides, containing straight horizontal bars of glass, having the section e c f. The light is not quite uniformly distributed by such apparatus, but the difficulty and expense attending the formation of prismatic rings were very great when Fresnel constructed this apparatus. Such rings can now be produced economically and accurately, and therefore the fixed-light apparatus is now constructed of circular glass rings, mounted in sections in such a manner that a vertical section through the axis of the apparatus would cut them in the form represented at e c f. Instead of forming the metal framework in which the glass is mounted with vertical ribs, it is made with the ribs placed somewhat diagonally, in order that the dark sectors which would be produced by the shadows of upright ribs may be avoided. It should be understood that the forms of the glass in each side of the octagonal apparatus represented in the figure are produced by the revolution of the same section, e c f, about the horizontal axis, d g.
An ingenious promoter of the catoptric system has contrived to solve the same problem by mirrors. The form of these may be understood by the aid of Fig. [306], which, however, relates to another contrivance. Suppose that the lines A B, A´ B´, are turned about C D as an axis, all three preserving their relative positions, A B and A´ B´ would sweep out two parabolic cones, which would have the property of reflecting in a horizontal direction all rays falling upon them from a lamp placed at L. But glass, as a material for lighthouse apparatus, has so many advantages over metal that it is probable that metallic reflectors will soon be entirely obsolete. The polish of the metal is very readily destroyed, and as it is constantly liable to be tarnished, the frequent cleaning required is apt to produce a scratched state of the surface, even when great care is used. Far greater accuracy of form can be imparted to glass than to metal reflectors. And then there is the great loss of light occurring at even the most highly polished surfaces of metal: a loss which is far greater than that occasioned by the refraction and reflections of the glass apparatus. There are cases, however, in which it is desirable to throw the whole of the light into one beam, and this cannot be done without reflecting the light from one side. Mr. Alan Stevenson contrived an excellent apparatus for this purpose, and the diagram, Fig. [306], will explain its nature. L is a point representing the source of light, A B, B´ A´, a parabolic metallic mirror. All the rays between L A and L B, and all between L A´ and L B´—that is, all those which fall upon the mirror—will be reflected parallel to L G; but those between L B and L B´ would escape from the mouth of the mirror, B B´, as a diverging cone. This is prevented by placing the lens, H I, the focus of which is at L, so as to convert the diverging cone, I L H, into the cylindrical beam, E H I F; and thus half the light emitted from the luminous point is sent in one direction. A hemispherical reflector, C K D, of which L is the centre, receives the other half, which is thus thrown back through L, and then follows the same course as the direct rays. For the metallic reflector, C K D, Mr. Stevenson afterwards substituted a system of glass zones; of which O P Q represents the sections. These had the same effect as the metallic reflectors, without the loss of light occasioned by the latter. The inner surface of the glass, C K D, is hemispherical, and the prismatic zones are such as would be produced by turning the section about L K (or C D) as an axis. The dotted lines show the course of a ray of light, L m, which, meeting the hemispherical surface perpendicularly, passes straight through it, and is totally reflected at m by the inclined surface, and again at n, so that it returns to L by the path n L. Reflecting glass prisms were also substituted for the metallic mirror, A B, B´ A´, and thus the use of metal has been entirely dispensed with in this apparatus. This light has been termed by Mr. Stevenson the holophotal (ὁλο, entire, φως, light). Such an apparatus will form the intensest beam that a given source of illumination can yield. On the other hand, when a fixed light is distributed to the whole horizon simultaneously, the illuminating power of the source is taxed to the utmost. These two cases may be considered the extreme modes of disposing of the light, while the parcelling of it into several beams, as effected by the apparatus represented in Fig. [305], is an intermediate mode.
