CHAPTER III.

THE ILLUMINATING APPARATUS OF LIGHTHOUSES.

It has very justly been said that the object of placing in a lighthouse an illuminating apparatus is, that, whether it be constructed of glass or of metal, it may _bend the rays_ (which would otherwise and naturally proceed in straight lines), and illuminate a hollow sphere, so that those rays which would otherwise be thrown upon the sky, and thereby wasted, may be made to fall on points at sea, where they will be clearly visible. If the light is to be a fixed one, intended to be seen all round, and from the horizon to the base of the light-tower, the upper rays issuing from an illuminating apparatus must be directed downwards, and the lower rays upwards, so as to increase the illumination. If it is desired to light up a narrow belt of the sea, extending from the horizon to the base of the lighthouse, all the rays must be bent laterally; or they may all be concentrated and thrown upon one or more spots of larger or smaller size, according as the light may be needed—as in the case of fixed lights placed at the end of narrow channels, and of revolving lights which are made visible all round by causing the lenses and reflectors to revolve about the source of light, or with that source about a centre.[17]

[17] _Edinburgh Review_, Jan. 1862, pp. 178, 179.

Two methods have been employed for the purpose of throwing light in the desired direction: first, by silvered parabolic reflectors, which is called the _Catoptric System_; second, by the employment of lenses of a peculiar construction, which is known as the _Dioptric_ (or _Refracting_) _System_.

Occasionally these two systems are combined, as in the ordinary _Catadioptric_, and in Mr. Stevenson’s admirable _Holophotal_ arrangement, whether Catoptric or Dioptric.

Before describing them, however, it will be desirable to offer a brief history of lighthouse illumination.

It was at a comparatively recent epoch that wood and coal fires were for the first time replaced by candles, and the open summit of the tower covered in with glass. About the end of the eighteenth century, for these insufficient producers of light, lamps were substituted, whose lustre was directed to a distance by reflectors of polished metal. Many of the lighthouses of this epoch were provided with the species of apparatus here described; among others, those of Capes de l’Ailly and de la Hève, the isles Rhé and Oléron. In 1782, an identical mode of lightage was established at Cordouan; but though this lighthouse did not include less than twenty-four lamps, accompanied each by a reflector, it diffused so feeble a light, that the seamen immediately insisted on a return to the barbarous system of the Middle Ages.

The apparatus of which they complained was, in truth, exceedingly defective; its lamps, differing but little from those which the seven foolish virgins suffered to die out, had broad wicks, and if they produced but little light, by way of compensation they emitted an enormous amount of smoke. It was natural, therefore, that men of science should, with a view to improvement, first direct their attention to the _lamp_. The pioneer in this course of inquiry was Argand, who, about 1748, contrived to secure “a double current of air;” which consists, as any one may see in the first lamp he meets with, of a wick, shaped like a hollow cylinder, enclosed in a glass tube. The heat caused by the combustion of the oil produces a vigorous draught, which leads to an abundant circulation of air both internally and externally; and air is for the lamp, as for man, the plant, and the animal—life!

Various modifications and improvements of Argand’s system have been successively introduced. The glass tube, for instance, by one inventor, was contracted at a short distance above the burner, so as to project more immediately the current of air upon the flame, and stimulate combustion. In his turn, Carcel bethought himself of supplying the wick with a superabundant quantity of oil, so as to avoid the heating of the burner, and to render the flame more regular; he thus succeeded in keeping the lamps burning for a longer period without a replenishment of the wick.

There then remained the _reflectors_. Curved in the form of a spherical segment, these received but a small portion of the luminous rays, and rarely returned them in the proper direction. Teulère, the engineer-in-chief of the province of Bordeaux, who was to distinguish himself at a later period by the erection of the Cordouan Tower, was ordered to make an examination of both the lamps and the reflectors, and to study the best means of remedying the evils complained of. His studies resulted in a paper of great interest, published in 1783. To concentrate in a single direction a large portion of the rays which were lost on all sides, he proposed the use of mirrors of perfect polish and a better form. By causing these mirrors to revolve around a lamp—that is, by projecting successively towards every point of the horizon the lustre formed by a large portion of the rays thus collected into a single sheaf—he invented at the same time the eclipse.

It was not at Cordouan, nevertheless, that the system was first applied, but at Dieppe, where the celebrated Borda, having studied Teulère’s paper, had a small revolving apparatus of five parabolic reflectors made[18] in 1784. The apparatus of Cordouan, likewise established by Borda, was not placed in the lighthouse tower until after its restoration by Teulère—that is, in 1790.

[18] To be more exact than have been the majority of authors who have written on lighthouses, we must add that a small revolving apparatus, with three reverberators (probably with spherical shells), had been planted at the mouth of the port of Marstrand, in Sweden, prior to 1783. The French engineer, however, had thought out the invention for himself, in ignorance that it had been elsewhere realized, and his was the merit of imagining a system so complete and so rational in all its parts, that nothing has since been added to or taken from his conception.

M. Leonel Reynard informs us that we are likewise wrong in attributing to Argand the idea of a lamp with a double current of air. It is to Teulère that it should, in the main, be attributed. However, this engineer, who has asserted the priority of his claim to the invention of the reflectors, and the system of eclipses, has not insisted upon that of the lamp. He limits himself to saying that Argand entertained the same idea as himself, and derived great profit from it.

* * * * *

This method of lightage was obviously a great improvement, and all the maritime powers hastened to adopt it. As the _Catoptric System_, it was, until within the last few years, exclusively employed on the coast of England. Though less esteemed in France, its use has not been entirely abandoned; and the French still employ catoptric apparatus for “the illumination of narrow channels, or for harbour-lights; to strengthen in a given direction a light whose range is sufficient for the maritime horizon generally; to illuminate lightships; and for service as provisional appliances.”

In the accompanying design we represent a plan and elevation of a catoptric apparatus, which is composed, as will be seen, of nine reflectors arranged in groups of threes. A small rotatory machine sets the system in motion, and eclipses at greater or shorter intervals are obtained by the varying speed with which it is worked. The range of the apparatus depends partly on its power, and partly on its position.

The reflectors, as used in the best lighthouses, are made, says Mr. Stevenson,[19] of sheet copper, plated in the proportion of six ounces of silver to sixteen ounces of copper. They are moulded to a paraboloidal form by a delicate and laborious process of beating with mallets and hammers of various forms and materials, and are frequently tested during the operation by the application of a mould carefully formed. After being brought to the curve, they are stiffened round the edge by means of a strong bizzle, and a strap of brass which is attached to it for the purpose of preventing any accidental alteration of the figure of the reflector. Polishing powders are then applied, and the instrument receives its latest finish.

[19] Stevenson, “On Lighthouses,” pp. 92, 93.

To prove the form of the reflector, two gauges of brass are employed. One is for the back, and used by the workmen during the process of hammering; the other—while the mirror undergoes its final touches—is applied to the concave face. The mirror is then tested by trying a burner in the focus, and measuring the intensity of the light at various points of the reflected conical beam.

The flame generally used in reflectors is derived from an Argand lamp, with wicks an inch in diameter. The burners are sometimes tipped with silver to prevent the wick from being wasted by the great heat which is evolved. They are also fitted, in many of the Scottish lighthouses, with a sliding apparatus of accurate shape, by which they can be removed from the interior of the mirror at cleaning time, returned exactly to the same place, and locked by means of a key.

* * * * *

Catoptric lights, we may add, are divided into _nine_ separate classes, differing in some respect from those recognized by the French authorities. The nine classes are called _fixed_, _revolving white_, _revolving red and white_, _revolving red with two whites_, _revolving white with two reds_, _flashing_, _intermittent_, _double fixed lights_, and _double revolving white lights_.

The following account of the distinctive character of each class of light is condensed from a valuable treatise by Mr. Alan Stevenson:—

The _fixed_ exhibits a regular and steady appearance, and is not subject to any change; and the reflectors employed are smaller than those required for revolving lights. This is necessary, in order that they may be ranged round the circular frame, with their axes so inclined as to admit of their illuminating every point of the horizon.

The _revolving light_ is produced by the revolution of a three or four sided frame, having large reflectors grouped on each side, with their axes parallel; and as the revolution exhibits once a minute, or once in two minutes, a light gradually increasing to the maximum, and then just as gradually decreasing to total darkness, its effect is remarkably impressive.

The _revolving red and white_ is obtained by the revolution of a frame whose different sides present red and white lights, and exhibit the following succession:—two white lights after one red, or two red lights after one white.

The _flashing light_ is effected in the same manner as the revolving; but, owing to a different construction of the frame, the reflectors on each of the eight sides are arranged with their rims or faces in one vertical plane, and their axes in a line inclined to the perpendicular—a disposition of the mirrors which, together with the greater quickness of the revolution, showing a flash once in five seconds of time, produces an impressive effect, wholly different from that of a revolving light, and presenting the appearance of an alternating rising and sinking illumination. The brightest and darkest periods being but momentary, this light is also characterized by a rapid succession of bright flashes; whence its name.

The _intermittent light_ is distinguished by bursting suddenly into view and continuing steady for a short time, after which it is suddenly eclipsed for half a minute. This is due to the perpendicular motion of circular shades in front of the reflectors, by which the light is alternately revealed and hidden.

The _double lights_ (“which are seldom used except where exists a necessity for a _leading_ line, as a guide for taking some channel or avoiding some danger”) are generally exhibited from two towers, one of which is higher than the other. At the Calf of Man, says Mr. Stevenson,[20] a striking variety has been introduced into the character of leading lights, by substituting for two _fixed_ lights, two lights which revolve in the same periods, and exhibit their flashes at the same instant; and these lights are, of course, susceptible of the other variety enumerated above, that of the revolving red and white lights, or flashing lights, coming into view at equal intervals of time. The utility of all these distinctions is to be estimated with reference to their property of at once striking the eye of an observer, and being instantaneously obvious to strangers.

[20] Stevenson, “On Lighthouses,” pp. 105-107.

The introduction of _colour_ as a source of distinction, is the only means of obtaining a sufficient number of varieties. Yet, in itself, it is an evil of no small magnitude. The effect being produced by interposing coloured media between the burner and the eye of the observer, much light is lost by the absorption of those rays which are retained in order to produce the desired appearance. Experiments have been made with almost every colour; but only red, blue, and green have proved useful, and the two latter merely at such short distances as to unfit them for “sea-lights.” Owing to the depth of tint required to produce a marked effect, the red shades generally absorb about from six-sevenths to five-sevenths of the whole light; a loss so immense as certainly to discourage their adoption whenever it can possibly be avoided. The red glass used in France absorbs only four-sevenths of the light; but then, as might be expected, its colour produces a much less signal distinction to the seaman’s eye. In some of the British lighthouses, the lights are very simply and conveniently coloured, by the use of chimneys of red glass, instead of placing large discs in front of the reflectors.

We come now to the _Dioptric_[21] _System of Lights_.

The application of lenses to lighthouses seems to have been proposed in England, and essayed at the South Foreland, as far back as 1752; but owing to mechanical imperfection, they were found to give a light inferior to that of the paraboloidal reflectors, and consequently were abandoned. Buffon, the great naturalist, suggested that a lens might be constructed in concentric zones out of a solid piece of glass; but the difficulties of the process have proved too great to be overcome. In, or about 1773, Condorcet proposed that burning lenses should be built up in separate pieces; and a similar method was described by Sir David Brewster in 1811. The same construction was quite independently discovered by the ingenious Fresnel in 1819; and soon afterwards he constructed a lens, placed a powerful lamp in its focus, and rendered it available for the practical purposes of a lighthouse. He is therefore the author, if not the inventor, of the highly successful system of illumination which bears his name.

[21] From the Greek δίοπτρον, an optical instrument with tube for looking through. Δίοπτρον is from διἁ, through; and ὅπτομαι, I see.

But before entering into a minuter description of the work, let us learn a few particulars of the man.

Jean Augustin Fresnel was born at Broglie, near Bernay, in the French department of the Eure, on the 10th of May 1788. When eight years old the future savant was still ignorant of his letters; a fact, says one of his biographers, to be attributed not so much to his delicate constitution as to a deep-rooted dislike for the study of languages, and, in general, for all exercises dependent upon the memory. But, on the other hand, at nine years of age, he was already distinguished by the experimental researches he had made in the domain of physics; which induced his parents to send him to the Polytechnic School. Here, rising step by step with remarkable rapidity, he eventually became Engineer of Ports et Chaussèes.

In 1819 he carried off the prize proposed by the Academy of Sciences on the difficult question of the diffraction of light. His investigations had long been directed to optical subjects, and hence, when the French Government established the Lighthouse Commission, Arago, who was nominated president, immediately appointed Fresnel to the important post of secretary.

Fresnel recognized the peculiar advantages of a plano-convex lens to refract in lines nearly parallel to their axis all the rays emanating from their foci. Like Condorcet and Brewster, who, as we have seen, had also turned their attention to the problem, though only for so far as concerned burning instruments, he asked himself whether, by arranging the lenses in stages, it was not possible to correct their spherical aberration—a defect which becomes all the more signal as the size of the lenses is enlarged—and, consequently, to obtain full command over the rays of a lamp.

* * * * *

Let us now transport ourselves to the upper story of a lighthouse, and putting aside the motive mechanism of the apparatus, let us penetrate into the lantern. Cast your glance upon the interior of that immense diamond which we call a _dioptric apparatus_. The first object which strikes our attention is the lamp. As the fire which shone on the summit of the edifice was the soul of the pharos, so the lamp is the soul of the modern lighthouse. It was to this lamp Teulère first directed his attention, when he brought the catoptric system to perfection; and it was to this lamp that Arago and Fresnel addressed themselves when engaged, in their time, in improving the work of Teulère, Argand, and Borda. Only, every lighthouse does not employ the same kind of lamp. In one, we meet with the Carcel lamp, where the oil is elevated to the wick by a clock-work mechanism. In another, it is the Moderator, in which the same function is discharged by a heavy weight surrounding a roller. In others, whose range is limited, it is the Permanent-level lamp, where the reservoir of oil is placed by the side and on the level of the burner, which possesses the power of regulating the supply.

Let us draw near, however, and carefully examine the lamp now before us, because in several details it differs from those we have described. It will specially interest us as an English invention.

At the epoch when Teulère and Argand had made the progress already specified in the construction of a suitable lamp, Rumford, desirous of effecting a still greater improvement, asked himself whether, by adapting it to burners with several concentric wicks, it would not be possible to increase its power of illumination. The attempt was made, but did not prove successful; he experienced considerable difficulty in regulating the flame of these multiple wicks, and in preventing their carbonization under the action of the intense heat developed by their combination. It was the study of this question which led Fresnel and Arago to their beautiful experiments on the illumination of lighthouses.

After repeated essays, these two men of science decided on the type of the lamp which we are now contemplating; an instrument remarkable not only for the whiteness and intensity of its light, but also for what I may call its power of endurance; it will burn for upwards of twelve hours without requiring to be touched. And that this advantage is most important the reader will apprehend, when he remembers that the lighthouse-flame must be kept kindled throughout the longest nights of winter.

* * * * *

At the present day, lighthouses of the third class are illuminated by lamps with two concentric wicks; which, in a certain sense, means two lamps in one. In lighthouses of the second class, each lamp has three; and in those of the first class, each has four wicks. In the latter we obtain, with a single illuminating apparatus, the full power of twenty-three Carcel lamps. The luminous focus, though gifted with so much potency, presents, nevertheless, but a flame of moderate breadth, and its light is as white as it is brilliant.

The oil employed in the lighthouses of Great Britain, Ireland, and France is the colza, which has of late years entirely superseded spermaceti oil, as producing an equal quantity of light at little more than half the expense. The electric light has, however, been proposed as a more powerful method of illumination. One system, in which the light is produced between carbon points by the revolution of magnets fixed on wheels, worked by a steam-engine, was tried with much success by Professor Holmes at the South Foreland; and is still, we believe, in use at Dungeness, as it is, in France, at the two lighthouses of La Hève. In the latter case, the mechanism producing the currents is composed of two steam-engines, each with a five-horse power, and of four electro-magnetic machines of six discs, composed each of sixteen bobbins. It is placed in a boat, adapted for the purpose, at an equal distance from the two towers. Under ordinary atmospheric conditions, a single steam-engine is kept in motion, communicating with a magneto-electric machine for each lighthouse. During fogs and mists, both engines are in activity, and each lighthouse receives the currents of two magneto-electric batteries, which are then associated.

Both lighthouses are supplied with two lenticular apparatus, placed one above another in the same lantern. The regulators of the progress of the carbons were invented by M. Serrin, whose object has been to augment their sensibility, and, consequently, the regularity of the light; in which respect, now-a-days, little is left to be desired. The mean intensity of the light produced by a machine of six discs is computed as equal to 200 Carcel burners. The intensity of the cone of light emanating from the lenticular apparatus, when illuminated in this manner, rises to 5000 burners.

The electric light, as yet, is applied only to lighthouses with fixed lights, for a special arrangement would be necessary in the lenticular apparatus before it could be employed with the same advantage in the production of intermittent lights (_feux à éclipses_). Experiments, however, have been made in this direction, which promise good results. Yet, in the present state of its mechanical conditions, the system of electric illumination does not seem susceptible of any very great development upon our shores. It cannot be applied economically to lights which require no very great intensity,—and these lights are the most numerous; and, on the other hand, the intricate constructions which it necessitates, the chances of accident which it presents, and the quantity of coal which it consumes, are obstacles to its employment in lighthouses isolated at sea, whose communications with the mainland are liable to interruption, and where it is of importance to reduce as much as possible the dimensions of the edifice as well as the amount of transport. However this may be, the electric light would seem destined to render valuable services to navigation at every point where it can be employed, and like the two great inventions which the history of marine lightage signalizes—that of paraboloidal reflectors, and next, that of the lenticular apparatus—it constitutes a special and noteworthy progress, under the threefold aspect of intensity of light, diversity of character, and the value of luminous unity.

We may add that Mr. Wilde, of Manchester, has invented a powerful electro-magnetic apparatus for lighthouse illumination, which may probably prove valuable. Modifications of the lime light, resulting from the action of an oxy-hydrogen flame upon a surface of prepared lime, have also been suggested; and the least powerful of these surpasses in brilliancy the best oil-lamp, as that surpasses the open coal-fire. We may, therefore, expect that as the latter barbarous mode of illumination gave way to the catoptric, or _reflecting_ system, so will the dioptric, before many years have passed, succumb to some ingenious apparatus capable of utilizing either the lime or the electric light.

* * * * *

We now return to Fresnel’s system, the _dioptric_, which is pretty generally adopted in the British lighthouses.

We must here premise that the system is based upon the laws of the refraction of light.

But, says the non-scientific reader, what do you mean by the _refraction_ of light? I know very well what reflection is; I am not so clear as to refraction.

A ray of light, when transmitted _obliquely_ from one transparent body to another of different density, undergoes, at the point where it strikes the common surface of the two planes, a sudden change of direction. This change of direction is called _refraction_. For instance, plunge one half of a straight ruler into a basin of water. The ruler no longer appears straight, but _bent back_ or _broken_ (_re_, and _fractum_) at the point where it enters the water.

We have already stated that the great object to be gained in lighthouse illumination is this very _refraction_; that is, the rays of the lamp must be refracted, or bent back, so as to strike and illuminate the sea.

Fresnel saw that this object might be secured by the employment of lenses to intercept, as it were, and refract the rays proceeding from the lamp. What kind of lens possessed the greatest power of refraction? He preferred the plano-convex lenses, which, instead of having two curve surfaces, have one surface a curve, and the other a plane. And the lens thus adopted he built up in separate pieces, for the still greater economization and intensity of light; and, says Mr. Stevenson, he has subdivided with so much judgment the whole surface of the lens into a centre lens and concentric annular bands, and has so carefully determined the elements of curvature for each, that it seems unlikely any improvement will soon be effected in their construction.

The central disc of the lens, marked B in the accompanying diagram—as employed in lights of the first order—is about 11 inches in diameter, and the focus distance equals 920 millimètres, or 36.22 inches. The annular rings surrounding it gradually decrease in breadth as they recede from the centre, from 2¾ to 1¼ inches. The lens, we should add, is made of crown glass.

A lens of this magnitude costs about £60. Its weight is about 109 lbs., and its surface consists of about 1300 square inches; but though composed of so many parts, it is held together simply by two narrow strips of polished glass, united by a thin film of cement.

* * * * *

The following illustration, representing a segment of the profile of a dioptric apparatus, will give the reader a sufficient idea of the manner in which the rays proceeding from the focus of a lamp are refracted on issuing from the lens; it also shows the central disc, and the rings placed above and below it.

When the drum is circular instead of being polygonal, the lenses are cylindrical and not annular; the luminous rays are uniformly distributed in the horizontal plane, and act—in a meridian section—in the same manner as those of the annular lenses.

Yet there is something more than lenses in a dioptric apparatus, for this reason, that the lamp does something more than illuminate the frame. The rays streaming below it vainly poured their light at the foot of the tower, and those which rose above it were diffused in the upper region of the atmosphere, and consequently, for all purposes of marine illumination, would have been useless, had not Fresnel conceived the idea of collecting, concentrating, and despatching them in the same direction as the lenses threw the others. This he effected by means of the cylindrical rings of glass which, above and beneath the lenses, cover over the framework, as it were, or make use of it as a base, in expanding themselves as they approach the centre of the apparatus.

The subjoined illustration represents the progress of a luminous ray in one of the rings of glass, technically termed _catadioptric rings_. Issuing from the focus F at the summit of the angle formed by the lines G and I, it is refracted at A in the direction A B, undergoes a complete reflection on the surface M N, takes the direction B C, and finally emerges from the ring in the horizontal line C H.

At this solution of the difficulty Fresnel did not arrive all at once, owing to the absolute want of workmen suitable for carrying out the novel industry which his genius had created. But by degrees these were trained and perfected; and the inventor had, moreover, the good fortune of discovering in an able and ingenious optician, M. Soleil, an efficient assistant in the construction on a large scale of the novel instrument he required. Afterwards the erection of lighthouses becoming an important branch of industry, he completed several edifices, which prospered all the more that strangers immediately gave up any attempt at rivalry, and left to him the work of supplying every maritime nation with lenticular apparatus.

* * * * *

Having said thus much of the central lens and its concentric rings of glass, a few words become necessary in reference to the lamp which feeds them, as it were, with light. Fresnel’s lamp may be shortly described as containing four concentric burners, which are defended from the excessive heat produced by their own combined flames by a superabundant supply of oil. This oil is pumped up from a cistern below by means of a clockwork movement, and overflows the wicks incessantly. To supply fresh currents of air to each wick with a rapidity sufficient to support the combustion, a very tall chimney-tube is found requisite. And yet the wicks do not carbonize with the extreme speed that might be supposed. It is even found, we are told, that after they have suffered a good deal, the flame does not perceptibly decrease, because the intense heat evolved from its mass encourages the rising of the oil in the cotton. Mr. Stevenson informs us that he has seen the large lamp in the Tour de Corduan burn for seven hours, and yet the wicks were neither snuffed nor raised. In the Scotch lighthouses a full flame is often maintained, with Colza oil, for no less a period than seventeen hours, and yet the lamp is untouched.

* * * * *

The only risk in using the Fresnel lamp, says Mr. Stevenson, arises from the liability to occasional derangement of the leathern valves that force up the oil by means of clockwork. Several lights on the French coast, and, more especially, the Tour de Corduan, have been extinguished by the failure of the lamp for a few minutes; an accident which has never happened, and scarcely _can_ happen, with the fountain lamps of the Catoptric system. To prevent such dangerous mishaps, which, under some circumstances, might entail the loss of a “tall ship,” various precautions have been adopted. The most efficacious seems to be this: an alarum is attached to the lamp, consisting of a small cup pierced in the bottom, which receives a portion of the oil overflowing from the wicks, and is capable, when full, of balancing a weight placed at the opposite end of a lever. The moment the machinery stops, the cup ceases to receive the supply of oil, and the remainder escaping at the bottom, the equilibrium of the lever is destroyed; it falls, and disengages a spring, which rings a bell with sufficient force to arouse a sleeping keeper. But, says Mr. Stevenson, shrewdly, it may justly be doubted whether such an arrangement might not actually tempt a keeper to relax in his vigilance, and rely on the alarum to waken him in case of need. In all the dioptric lamps on the British coast, therefore, the converse method is adopted of causing the bell to cease when the clockwork stops.

Another and more important precaution consists in keeping always at hand, in the light-room, a _spare lamp_, trimmed, and adjusted to the proper height for the focus, and in every respect ready to act as a substitute for the other if any accident occurs.

But while I am tracing these words, I read that experiments have been successfully made with _gas_ for the illumination of the lenticular apparatus, and that, if it will afford a steadier and fuller light, at less expense, and with no risk of accident, it will probably be adopted.

* * * * *

To continue:—

Once having acquired a full command of all the rays amplified from the lamp, the next desideratum was to diversify the appearance of the light which they constituted; for, as I have already said, it is not enough to stretch a belt of warning fires around the coast,—we must take care that each shall in some wise be distinguished from the other, so as to afford the navigator a clue to its particular locality. Hence arose the division into fixed, revolving, intermittent lights, and so on, which I have already described, and which is secured in the following manner:—

If a _fixed light_ be required, the apparatus as invented by Fresnel takes the form of an annular glass frame produced by the revolution of the section passing through the centre of a circular lens, and reflecting prisms around a vertical elevated on the principal axis of this section, as shown in the diagram.

For _revolving apparatus_, Fresnel’s apparatus, as employed in all lighthouses prior to the introduction of the holophotal arrangement, consisted, as may be seen in the diagram, of annular lenses, _L_, for acting on the central part of the light, while the upper rays were refracted by inclined hanging lenses, _a_, and ultimately reflected into the proper direction by silvered mirrors marked _b_, placed above. The lower rays were intercepted by fixed light prisms, _p p_ (_which did not revolve_), and which, showing a fixed light all round, were, of course, of very inferior power to the solid beams proceeding from the large lenses =L=, and the smaller lenses and mirrors placed above. Strictly speaking, Fresnel’s revolving light consisted of a _revolving_ and a weak _fixed_ light. As the frame revolved round the central lamp, the mariner saw the luminous beam when the lenses were turned towards him, and the number of flashes depended on the quickness of the rotation.

The apparatus adopted by Fresnel for the _fixed_ light may be regarded as perfect; but his _revolving_ light has been now superseded by the holophotal apparatus of Mr. Thomas Stevenson. The inclined mirrors and lenses employed in Fresnel’s apparatus are done away with in Mr. Stevenson’s, shown in the diagram, in which, by the single agency of lenses, _L_, and totally reflecting prisms, _p_, all the rays are rendered parallel. In this form the _whole_ glass frame, consisting of lenses and reflecting prisms, revolves round the central lamp. As Fresnel’s lighthouse prisms only gathered the light vertically, they could not produce the sheaf of rays required for the revolving light unless when combined with others which gathered the rays horizontally. The first lighthouse in which single prisms were made to revolve was the Horsburgh light, near Singapore, the apparatus of which was designed by Mr. Stevenson in 1850. In this form of revolving light apparatus the prisms are generated about a horizontal instead of a vertical axis, as in fixed light. The forms of the beams of light issuing forth from Fresnel and Stevenson’s apparatus are shown opposite to the diagram of each, and marked _x, y_.

In France there is frequently employed what Fresnel called a “_fixed light varied by flashes_.” This effect is produced, as already explained, by causing panels of glass, curved horizontally but not vertically, to revolve outside of Fresnel’s fixed apparatus, as shown in the diagram. The ordinary fixed apparatus only acts in the vertical plane, while the straight panels only act in the horizontal plane. So that when the fixed apparatus is alone visible the rays are only gathered from the vertical plane, and the light is comparatively weak; but when the panels come opposite the eye, the rays are gathered from both planes into one powerful beam, as in a revolving light. Here, as in the former case, two agents are employed, causing great loss of light and great unnecessary expense, where one, if of the proper form, is sufficient. Mr. Stevenson’s modification of the holophotal arrangement for this purpose is shown in the diagram. It consists of alternate panels of the fixed light and holophotal apparatus; and thus, by single agency, a weak fixed light and a stronger revolving light are shown time-about to the mariner, which is the required characteristic.

If, for the sake of further diversity, the lights are to be coloured, we content ourselves in the case of a fixed light, as already stated, with enclosing the flame in a green or red tube. In the French eclipsing apparatus, polished sheets of coloured glass are placed, on one side or the other, against the lenses intended to emit the flashes of colour.

While doing all honour to Fresnel and his great invention, we must not forget that its present comparative perfection is due to Mr. Thomas Stevenson, whose improvements, in truth, have eventuated in almost a new system, now known as the _Holophotal_,[22] and already partially described. By a peculiar combination of dioptric spherical mirrors and other apparatus, it also succeeds in economizing and condensing into one beam the whole of the rays thrown off from the burners; but I fear that any explanation of it which could here be attempted would, from its necessary introduction of technical language, prove unintelligible to the non-scientific reader.

[22] From ὅλος, entire; and φὼς light.