Chapter 27

[71]See also M.Leonor Fresnel’sInstructions sur l’organisation et la surveillance du service des Phares et Fanaux de France. Paris, 1842, pp. 12, 13, 14, and 15.

The focal point for the lenses and refractors is in the centre of the flame and on the level of its brightest film, as shewn inPlate XXV.Choice of Focal Point for various parts of the apparatus.The choice of a focus for the zones naturally formed a most important practical consideration in their arrangement; and the judicious remarks of M.Leonor Fresnelon that subject, already noticed, would alone have induced me to discard my former calculations in favour of his. For the upper zones, M.Fresnelhad adopted a point in the centre of the flame 10 millimètres above the focus of the lenses, so that all the lightbelowthat point necessarily falls between the horizon and the Lighthouse; but for the lower zones, it was necessary, owing to their arrangement for convenience in a cylindric form, to adopt a separate focus for each zone in the direction of the centre of gravity of that part of the flame which would light each zone. In this manner (fig. 86) the foci of the zones recede upwards fromatofin proportion to the depression of the zonesa,b,c,d,e,f, so that the line joining each zone and its focus, must revolve as aradius vectorround some point O between them. The details of this arrangement are shewn inPlate XVIII.; and are also given in theTableof the Catadioptric Zones in the Appendix.

Fig. 86.Foci of individual lenses

Fig. 86.

Fig. 87.Spherical mirror in dioptric light

Fig. 87.

In the arc next the land, in fixed lights, a great loss of light ensues from the escape of the rays uselessly in that direction. So far back as 1834, I suggested theApplication of Spherical Mirrors to fixed Dioptric Lights.placing a segment of a spherical mirror, with its centre of curvature coincident with F the focus of the system, so that the luminous pyramid MFM, of which the mirror MM forms the base, might be thrown back through the focal point and finally refracted into such a direction as to contribute to the effect of the lens QAqin seaward and opposite arc. In the diagram (fig. 87),rrindicate rays proceeding directly from F;r′r′rays reflected from MM through F, and finally refracted at QAq; andr″r″is the beam compounded of both. In the best glass-silvered mirrors, this accession of light would amount to nearly half of the light incident on them. In such an arrangement, a considerable radius is desirable to decrease the amount of aberration produced by a large flame. In the case of revolving lights of the first order, the radius would, of course, be limited to somewhat less than three feet, which is the focal distance of the lenses, between which and the focus, the reflecting segment must be placed; but in fixed lights, the lantern is the limit of radius, so that a focal length of five feet ten inches may be obtained. M.Françoisground some beautiful mirrors of three feet radius, which were afterwardstinnedby his successor, M.Letourneau, by a new process discovered by himself;[72]and that gentleman is at present engaged in the construction of reflecting spherical segments 1200mm.square (about 16 superficial feet), to a radius of 1770mm.(5 feet 10 inches), which subtend a vertical arc of about 40°.

[72]See notice of a similar process practised about the year 1750 by MrRogersof London,ante,p. 240.

Arrangement of Dioptric Apparatus.The arrangements of the dioptric apparatus in the lightroom will be more fully understood by referring to the Plates.

Plate XIII.shews an elevation of a revolving dioptric apparatus of the first order; F is the focal point, in which the flame is placed;L, L great annular lenses, forming by their union an octagonal prism, with the lamp in its axis, and projecting, in horizontal beams, the light which they receive from the focus; L′L′, the upper lenses, forming by their union a frustum of an octagonal pyramid of 50° of inclination, and having their foci coinciding in the point F. They parallelise the rays of light which pass over the lenses. M, M are plane mirrors, placed above the pyramidal lenses L′L′, and so inclined as to project the beams reflected from them in planes parallel to the horizon; Z, Z are the lower zones, first used at Skerryvore, in the room of the curved mirrors which were used at Corduan. The lower part ofPlate XIII.shews the moveable framework which carries the lenses and mirrors, and the rollers on which it circulates, with the clockwork which gives motion to the whole.Plate XIV.is the plan of the apparatus shewn inPlate XIII.

Plate XV.shews a section of a fixed dioptric light of the first order. F is the focal point in which the flame is placed; R, R cylindric refractors, forming by their union a prism of thirty-two sides, or a true cylinder, with the lamp in its axis, and producing a zone of light of equal intensity in every point of the horizon; M, M, curved mirrors, ranged in tiers above and below the cylindric refractors, and having their foci coinciding in the point F; the effect of the mirrors increases the power of the light, by collecting and transmitting the rays which would otherwise pass above and below them, without increasing the effect of the light.Plate XVI.gives the elements of the curved mirrors MM ofPlate XV.

After the details given of the nature of the catadioptric zones, all that is needful is briefly to refer toPlate XVII., in which ABC and A′B′C′ shew the upper and lower zones which supply the place of the mirrors shewn at M, M, inPlate XV.; while DEF shews the cylindric belt as lately improved, with the diagonal joints M, N, C; and X, X, represent the diagonal supports for the cupola ABC. This plate, in connection with the enlarged view of the same apparatus atPlate XVIII., affords a complete explanation of the arrangement of all the parts.

Arrangement of the Dioptric Apparatus in the Lightroom.For the purpose of arranging the various parts of the dioptric apparatus in their proper positions,threegauges are employed.The first (fig. 88) is for ascertaining that the lensesl,l, meet at the proper horizontal angle, so that their axes shall meet with the proper inclination in F the focus. This is done by means of two arms, whose projecting pointsr,r,r,rtouch the backs of the lenses, while the graduated arccindicates the inclination ofl,l, tol,l, or the complement of that inclination at F.

Fig. 88.Gauge

Fig. 88.

Fig. 89.Clinometer

Fig. 89.

Again, for ascertaining the verticality of the main lenses, or for setting the subsidiary lenses or mirrors shewn inPlates XIII.andXIV., at the required angle of inclination, recourse is had to a clinometer (fig. 89) touching the back of the lens LL by means of studs at A, A, while the spirit-level S indicates, on the graduated limb, the amount of deviation from the vertical position of the instrument, whether accidental or intentional.

Fig. 90.Gauge

Fig. 90.

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 thelensl,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 atcat 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 atpage 290, in speaking of the lamp. The dotted line Fc′B′A′ shews the position of the trainer in reference to an adjacent lens.

The elegant apparatus invented byAugustin Fresnelfor 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 bytotal reflection.

Fig. 91.Fresnel's harbour light apparatus

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 tofig. 91, which shews its section and plan. F is the focal point in which the flame is placed;r,rcylindric 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,xare catadioptric prismatic rings acting bytotal 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 lettersr′r′shew the external prisms,having their axes at right angles to those of the principal bent prisms, composing the refractors atr,r, and revolving around them. This ingenious application of the property of crossed prism is already described atpage 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 ofThilorier, which has a balance ofsulphate 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 sameproportionaldimensions. Its internal diameter, however, is 500 millimètres (about 19¹⁄₂ inches). Drawings of the smaller apparatus are given atPlate 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 theTableof the Great Zones, which will be found in theAppendix:—

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.

Orders of the French Lights.The dioptric lights used in France are divided into four orders, in relation to their power and range; but in regard to their characteristic appearances, this division does not apply, as, in each ofthe orders, lights of identically the same character may be found, differing only in the distance at which they can be seen, and in the expense of their maintenance. The four orders may be briefly described asfollows:—

1st, Lights of the first order having an interior radius or focal distance of 36·22 inches (92cm.), and lighted by a lamp of four concentric wicks, consuming 570 gallons of oil per annum.

2d, Lights of the second order having an interior radius of 27·55 inches (70cm.), lighted by a lamp of three concentric wicks, consuming 384 gallons of oil per annum.

3d, Lights of the third order, lighted by a lamp of two concentric wicks, consuming 183 gallons of oil per annum. The instruments used in those lights are of two kinds, one having a focal distance of 19·68 inches (50cm.), and the other of 9·84 inches (25cm.).

4th, Lights of the fourth order, or harbour-lights, having an internal radius of 5·9 inches (15cm.), and lighted by a lamp of one wick, or Argand burner, consuming 48 gallons of oil per annum. This apparatus is, as already noticed, now more generally used of a larger scale, having a focal distance of 9·84 inches (25cm.), and a lamp of two concentric wicks, consuming about 130 gallons of oil per annum.[73]

[73]An apparatus of 0m.·185cm.was recently added to the list of French lights under the name of theFifthorder; while that of 25cm.radius has been called theFourth, and that of 15cm.radius is styled theSixthorder. Those minute subdivisions I consider to be unnecessary.

Distinctions of the Dioptric Lights.Those four orders are not intended as distinctions; but are characteristic of the power and range of lights, which render them suitable for different localities on the coast, according to the distance at which they can be seen. This division, therefore, is analogous to that which separates our lights intosea-lights,secondary lights, andharbour-lights, terms which are used to designate the power and position, and not the appearance of the lights to which they are applied.

Each of the above orders is susceptible of certain combinations, which produce various appearances, and constitute the distinctions used for dioptric lights; but the following are those which have been actually employed as the most useful inpractice:—

The first order contains, 1st, Lights producing, once in every minute, a great flash, preceded by a smaller one, by the revolution of eight great lenses and eight smaller ones combined with eight mirrors; 2d, Lights flashing once in every half minute, and composed of sixteen half lenses. Those lights may have the subsidiary parts simply catoptric, or diacatoptric; and, 3d, Fixed lights, composed of a combination of cylindric pieces, with curved mirrors or catadioptric zones ranged in tiers above and below them.

The second order comprises revolving lights with sixteen or twelve lenses, which make flashes every half minute; and fixed lights varied by flashes once in every four minutes, an effect which, as already noticed, is produced by the revolution of exterior cylindrical pieces.

The third order (larger diameter) contains common fixed lights, and fixed lights varied by flashes once in every four minutes.

The third order (smaller diameter) contains fixed lights, varied by flashes once in three minutes.

The fourth order has fixed lights varied by flashes once in every three minutes, and fixed lights of the common kind. It has been thought necessary to change the term “fixed lights varied by flashes,” for “fixed light with short eclipses,” because it has been found that, at certain distances, a momentary eclipse precedes the flash.

These distinctions depend upon the periods of revolution, rather than upon thecharacteristic appearanceof the light; and therefore seems less calculated to strike the eye of a seaman, than those employed on the coasts of Great Britain and Ireland. In conformity with this system, and in consideration of the great loss of light which results from the application of coloured media, all distinctions based upon colour have been discarded in the French lights.

The distinctions are, in fact, onlyfourin number, viz.: Fixed; Fixed, varied by flashes;[74]Revolving, with flashes once a minute; and Revolving, with flashes every half minute. To those might be added, Revolving, with bright periods once in two minutes, and perhapsFlashingonce infive seconds(as introduced by me at the Little Ross, but I cannot say with such complete success as would induce me to recommend its general adoption). My own experience would also lead me to reject the distinction called “Fixed, varied by flashes,” which I do not consider as possessing a marked or efficient character.

[74]The “Feu fixe, varié par des éclats,” or “Feu fixe, à courtes éclipses,” of Fresnel.

Comparison of Dioptric and Catoptric Apparatus for Revolving Lights.Having thus fully described the nature of the catoptric and dioptric modes of illuminating lighthouses, I shall next compare the merits of both systems, with a view to determine their eligibility in revolving or in fixed lights.

Repeated experiments were made at Gullan-hill, which is distant from Edinburgh about fifteen miles, during the winters of 1832 and 1833, under the inspection of the Commissioners of Northern Lights, the result of which was, that the light of one of the great annular lenses used in the revolving lights of the first order, was equal to the united effect of about eight of the large reflectors employed in the revolving lights on the Scotch coast. It may be said, however, that the dia-catoptric[75]combination of pyramidal lenses and plane mirrors of Corduan, adds the power of more than two reflectors to the effect of the great lens; but it ought to be remembered that in the French lights, this additional power is used only to compensate for one of the defects of the system by lengthening the duration of the flash, and therefore contributes, if at all, only in a very indirect manner, to render the light visible to the mariner at a greater distance. M.Fresnelfound, from the smaller divergence of the lens, that the eclipses were too long and the bright periods of the revolution too short; and he therefore determined toadopt the horizontal deviation of 7° for the upper lenses, with a view to remedy this defect. Assuming, therefore, that it were required to increase the number of reflectors in a revolving light of three sides, so as to render it equal in power to a dioptric revolving light of the first order, it would be necessary to place eight reflectors on each face, so that the greatest number of reflectors required for this purpose may be taken attwenty-four. M.Fresnelhas stated the expenditure of oil in the lamp of four concentric wicks at 750 grammes of colza oil per hour; and it is found by experience at the Isle of May and Inchkeith, that the quantity of spermaceti oil consumed by the great lamp, is equal to that burned by from fourteen to sixteen of the Argand lamps used in the Scotch lights. It therefore follows that, by dioptric means, the consumption of oil necessary for between fourteen and sixteen reflectors, will produce a light as powerful as that which would require the oil of twenty-four reflectors in the catoptric system of Scotland; and, consequently, that there is an excess of oil equal to that consumed by ten reflectors, or 400 gallons in the year, against the Scotch system. But in order fully to compare the economy of producing two revolving lights of equal power by those two methods, it will be necessary to take into the calculation the interest of the first outlay in establishing them.

[75]I use this word to designate the arrangement of pyramidal lenses and plane mirrors, by which the light is firstrefracted, and thenreflected.

The expense of fitting up a revolving light with twenty-four reflectors, ranged on three faces, may be estimated at L.1298, and the annual maintenance, including the interest of the first cost of the apparatus, may be calculated at L.418, 8s. 4d. The fitting up a revolving light with eight lenses and the dia-catoptric accessory apparatus, may be estimated at L.1459, and the annual maintenance at L.354, 10s. 4d. It therefore follows, that to establish and afterwards maintain a catoptric light of the kind calledrevolving white, with a frame of three faces, each equal in power to a face of the dioptric light of Corduan, an annual outlay of L.63, 18s. more would be required for the reflecting light than for the lens light; while for a light of the kind calledrevolving red and white, whoseframe has four faces, at least thirty-six reflectors would be required in order to make the light even approach an equality to that of Corduan; and the catoptric light would in that case cost L.225 more than the dioptric light.

The effect produced by burning an equal quantity of oil, in revolving lights on either system, may be estimated as follows:—In a revolving light, like that of Skerryvore, having eight sides, each lighting with its greatest power a horizontal sector of 4°, we have 32° (orunits) of the horizon illuminated with the full power of 3200 Argand flames, and consequently an aggregate effect of 102,400 flames, produced by burning the oil required forsixteenreflectors; while in a catoptric apparatus, like that of the old light at Inchkeith, having seven sides of one reflector, each lighting with its greatest power a sector of 4°·25′, we have nearly 31° (orunits) of the horizon illuminated with the full power of 400 Argand flames, and consequently an aggregate effect of 12,400 flames as the result of burning the oil required forsevenreflectors. Hence, theeffectof burning the same quantity of oil in revolving lights on either system, will be represented respectively by16712,400 = 28,343 for the catoptric, contrasted with 102,400 for the dioptric light; or, in other words, revolving lights on the dioptric principle use the oil moreeconomicallythan those on the catoptric plan, nearly in the ratio of 3·6 to 1.

Comparison of Catoptric and Dioptric Apparatus for Fixed Lights.I shall now speak of fixed lights, to which the dioptric method is peculiarly well adapted. The effect produced by the consumption of a gallon of oil in a fixed light, with twenty-six reflectors, which is the smallest number that can be properly employed, may be estimated as follows:—Themeaneffect of the light spread over the horizontal sector, subtended by one reflector, as deduced from measurements made at each horizontal degree, by the method of shadows, is equal to 174 unassisted Argand burners. If, then, this quantity be multiplied by 360 degrees, we shall obtain an aggregate effect of 62,640, which, divided by 1040 (the number ofgallons burned during a year intwenty-sixreflectors), would give 60 Argand flames for the effect of the light maintained throughout the year by the combustion of a gallon of oil. On the other hand, the power of a catadioptric light of the first order, like that lately established at Girdleness, may be estimated thus:—Themeaneffect of the light produced by the joint effect of both the dioptric and catadioptric parts of a fixed light apparatus, may be valued at 450 Argand flames, which, multiplied by 360 degrees, gives an aggregate of 162,000; and if this quantity be divided by 570 (the number of gallons burned by the great lamp in a year), we shall have about 284 Argand flames for the effect of the light produced by the combustion of a gallon of oil. It would thus appear that in fixed lights, the French apparatus, as lately improved, produces, as theaverageeffect of the combustion of the same quantity of oil over the whole horizon, upwards offour timesthe amount of light that is obtained by the catoptric mode; although, in certain directions, opposite the axis of each reflector, the catoptric light be fully 50per centummore powerful than the dioptric light.

But the great superiority of the dioptric method chiefly rests upon itsperfectfulfilment of an important condition required in a fixed light, by distributing the raysequallyin every point of the horizon. In the event of the whole horizon not requiring to be illuminated, the dioptric light would lose a part of its superiority in economy, and when half the horizon only is lighted, it would be more expensive than the reflected light; but the greater power and more equal distribution of the light, may be considered of so great importance, as far to outweigh the difference of expense. In the latter case, too, an additional power (as noticedp. 293) can be given to the dioptric light, by placing at the landward side of the lightroom, spherical mirrors with their centres in the focus of the refracting apparatus.[76]The luminous cones, or pyramids of whichsuch reflectors would form the bases, instead of passing off uselessly to the land, would thus be thrown back through the focal point, and finally refracted, so as to increase the effect of the light seaward, by nearlyone-thirdof the light which would otherwise be lost.

[76]A similar arrangement can also be made in revolving lights by making the radius of the mirrors somewhat less than that of the inscribed circle of the octagon bounded by the lenses, so that they may circulate freely round the backs of the mirrors. The shortness of the radius of the reflecting surface would, of course, increase the divergence of the beam of light refracted through the lenses, as the flame would, in this case, subtend a greater angle at the face of the mirrors.

The expense of establishing a fixed light composed of twenty-six reflectors, may be estimated at L.950, and its annual maintenance, including interest on the first cost of the apparatus, may be reckoned at L.425, 10s.: and the expense of fitting up a fixed light on the dioptric principle with catadioptric zones is L.1511, while its annual maintenance may be taken at L.285, 6s. 4d. It thus appears that the annual expenditure of the dioptric fixed light is L.140, 3s. 8d. less than that of a fixed light composed of twenty-six reflectors; while theaverageeffect, equally diffused over the horizon, isfour timesgreater.

The comparative views already given of the catoptric and dioptric modes of illuminating lighthouses, demonstrate that the latter produces more powerful lights by the combustion of the same quantity of oil; while it is obvious that the catoptric system insures a more certain exhibition of the light, from the fountain-lamps being less liable to derangement than the mechanical lamps used in dioptric lights. The balance, therefore, of real advantages or disadvantages, and, consequently, the propriety of adopting the one or the other system, involves a mixed question, not susceptible of a very precise solution, and leaving room for different decisions, according to the value which may be set upon obtaining a cheaper and better light, on the one hand, as contrasted, on the other, with less certainty in its exhibition.

Summary of considerations as to the fitness of the two systems for Revolving Lights.A few general considerations, serving briefly to recapitulate the arguments for and against the two systems, may not be out ofplace. And, first, regarding the fitness of dioptric instruments for revolving lights, it appears from the details abovegiven,—

1st, That by placing eight reflectors on each face of a revolving frame, a light may be obtained as brilliant as that derived from the great annular lens; and that, in the case of a frame of three sides, the excess of expense by the reflecting mode, would be L.63, 18s.; and in the case of a frame of four sides, the excess would amount to L.225.

2d, That for burning oil economically in revolving lighthouses, which illuminate every point of the horizon successively, the lens is more advantageous than the reflector in the ratio of 3·6 to 1.

3d, That the divergence of the rays from the lens being less than from the reflector, it becomes difficult to produce, by lenses, the appearance which characterises the catoptric revolving lights, already so well known to British mariners; and any change of existing lights which would, of course, affect their appearance, must, therefore, involve many grave practical objections which would not at all apply to the case of new lights.

4th, That the uncertainty in the management of the lamp renders it more difficult to maintain the revolving dioptric lights without risk of extinction, an accident which has several times occurred at Corduan and other lighthouses both in France and elsewhere.

5th, That the extinction of one lamp in a revolving catoptric light is not only less probable, but leads to much less serious consequences than the extinction of the single lamp in a dioptric light; because, in the first case, the evil is limited to diminishing the power ofone faceby aneighth part; whilst, in the second,the whole horizon is totally deprived of light. The extinction of a lamp, therefore, in a dioptric light, leads to evils which may be consideredinfinitely greatin comparison with the consequences which attend the same accident in a catoptric light.

Summary of considerations as to the fitness of the two systems for Fixed Lights.In comparing the fixed dioptric, and the fixed catoptric apparatus, the results may be summed up under the followingheads:—

1st, It is impossible, by means of any practicable combination of paraboloïdal reflectors, to distribute round the horizon a zone of light of exactlyequal intensity; while this may be easily effected, by dioptric means, in the manner already described. In other words, the qualities required in fixed lights cannot be so fully obtained by reflectors as by refractors.

2d, Theaveragelight produced in every azimuth by burning one gallon of oil in Argand lamps, with reflectors, is only aboutone-fourthof that produced by burning the same quantity in the dioptric apparatus; and the annual expenditure is L.140, 3s. 8d. less for the entire dioptric light than for the catoptric light.

3d, Thecharacteristicappearance of the fixed reflecting light in any one azimuth would not be changed by the adoption of the dioptric method, although its increasedmeanpower would render it visible at a greater distance in almost every direction; the only exception being in the azimuths opposite the axis of each reflector, where the catoptric light has anexcessof power equal to about 50per centum.

4th, From the equal distribution of the rays, the dioptric light would be observed at equal distances in every point of the horizon; an effect which cannot be fully attained by any practicable combination of paraboloïdal reflectors.

5th, The inconveniences arising from the uncertainty which attends the use of the mechanical lamp, are not perhaps so much felt in a fixed as in a revolving light, because the greater simplicity of the apparatus admits of easier access to it, in case of accident.

6th, But the extinction of a lamp in a catoptric light, leaves only onetwenty-sixth partof the horizon without the benefit of the light, and the chance of accident arising to vessels from it, may, therefore, be considered as incalculably less than the danger resulting from the extinction of the single lamp of the dioptric light, which deprives the whole horizon of light.

7th, There may also, in certain situations, be some risk arising from irregularity in the distances at which the same fixed catoptriclight can be seen in the different azimuths. This defect, of course, does not exist in the dioptric light.

Advantages and disadvantages of both systems under certain circumstances.There can be little doubt, that the more fully the system ofFresnelis understood, the more certainly will it be preferred to the catoptric system of illuminating lighthouses, at least in those countries where this important branch of administration is conducted with the care and solicitude which it deserves. It must not, however, be imagined, that there are no circumstances in which the catoptric system is not absolutely preferable to illumination by means of lenses. We have hitherto attended only to horizontal divergence and its effects, and this is unquestionably the more important view; but the consideration of vertical divergence must not be altogether overlooked. Now, while it is obvious that vertical divergence, at least above the horizon, involves a total loss of the light which escapes uselessly upwards into space, it is no less true, that if the sheet of light which reaches the most distant horizon of the lighthouse, however brilliant, were as thin as the absence of all vertical divergence would imply, it would be practically useless; and some measure of dispersion in the arcbelowthe horizon is therefore absolutely indispensable to constitute a really useful light. In the reflector, the greatest vertical divergence below the horizontal plane of the focus is 16° 8′, and that of the lens is about 4° 30′.

Let us consider for a moment the bearing of those facts upon the application of the two modes of illumination to special circumstances. The powerful beam of light transmitted by the lens, peculiarly fits that instrument for the great sea-lights which are intended to warn the mariner of his approach to a distant coast which he first makes on anover-seavoyage; and the deficiency of its divergence, whether horizontal or vertical, is not practically felt as an inconvenience in lights of that character, which seldom require to serve the double purpose of being visible at a great distance, and at the same time of acting as guides for dangers near the shore. For such purposes, the lens applies the light much more economicallythan the reflector, because, while the duration of itsleastdivergent beam is nearly equal to that of the reflector, it iseighttimes more powerful. A revolving system of eight lenses illuminates an horizontal arc of 32° with this bright beam. The reflector, on the other hand, spreads the light over a larger arc of the horizon; and, while itsleastdivergent beam is much less powerful than that of the lens, the light which is shed over itsextremearc is so feeble as to be practically of little use in lights of extensive range, even during clear weather. When a lighthouse is placed on a very high headland, however, the deficiency of divergence in the vertical direction is often found to be productive of some practical inconvenience; but this defect may be partially remedied by giving to the lenses a slight inclination outwards from the vertical plane of the focus, so as to cause the most brilliant portion of the emergent beam to reach thevisible horizonwhich is due to the height of the lantern. It may be observed, also, that a lantern at the height of 150 feet, which, taking into account the common height of the observer’s eye at sea, commands a range of upwards of 20 English miles, is sufficient for all the ordinary purposes of the navigator, and that the intermediate space is practically easily illuminated, even to within a mile of the lighthouse, by means of a slight inclination of the subsidiary mirrors, even where the light from the principal part of the apparatus passes over the seaman’s head. For the purpose of leading lights, in narrow channels, on the other hand, and for the illumination of certain narrow seas, there can be no doubt that reflectors are much more suitable and convenient. In such cases, the amount of vertical divergence below the horizon, forms an important element in the question, because it is absolutely necessary that the mariner should keep sight of the lights even when he is very near them; while there is not the same call for a very powerful beam which exists in the case of sea-lights. Yet even in narrow seas, where low towers, corresponding to the extent of therangeof the light, are used, but where it is, at the same time, needful to illuminate thewhole or the greater part of the horizon, the use of dioptric instruments will be found almost unavoidable, especially in fixed lights, as well from their equalizing the distribution of the light in every azimuth, as from their much greater economy in situations where a large annual expenditure would often be disproportionate to the revenue at disposal. In such places, where certain peculiarities of the situation require the combination of a light equally diffused over the greater portion of the horizon, along with a greater vertical divergence in certain azimuths, than dioptric instruments afford, I have found it convenient and economical to add to the fixed refracting apparatus a single paraboloïdal reflector, in order to produce the desired effect, instead of adapting the whole light to the more expensive plan for the sake of meeting the wants of a single narrow sector of its range. In other cases, where the whole horizon is to be illuminated, and great vertical divergence is at the same time desirable, a slight elevation of the burner, at the expense, no doubt, of a small loss of light, is sometimes resorted to, and is found to produce, with good effect, the requisite depression of the emergent rays.

In certain situations, where a great range and, consequently, a powerful light must be combined with tolerably powerful illumination in the immediate vicinity of the lighthouse, we might, perhaps, advantageously adopt a variation of the form and dimensions of the mirrors employed, so as to resemble those formerly employed at the Tour de Corduan, which were of considerably larger surface and longer focal distance than those which are used in Britain. If such a form were adopted, the power of the light for the purpose of the distant range would be increased; and I would propose to compensate for the deficiency of divergence consequent on a long focal distance, by placing a second burner in some position between the parameter and the vertex, and slightly elevated above the axis of the instrument, so as to throw the greater portion of the beam resulting from this second burner below the horizontal plane of the focus. Such an expedient is no doubt somewhatclumsy and would at the same time involve the consumption of twice the quantity of oil used in an ordinary catoptric light; but I can still conceive it to be preferable, in certain situations, to the use of lenses alone.

Thus it appears that we must not too absolutely conclude against one, or in favour of the other mode of illumination for lighthouses; but, as in every other department of the arts, we shall find the necessity of patiently weighing all the circumstances of each particular case that comes before us, before selecting that instrument, or combination of instruments, which appears most suitable.

Distinctions of the Dioptric Lights, and the application of coloured media.The mode of distinguishing lights in the system ofFresnel, depends more upon theirmagnitudeand themeasured intervalof the time of their revolution, than upon theirappearance; and no other very marked distinctions, except Fixed and Revolving, have been successfully attempted in France. As above stated, I consider the distinction of thefixed light varied by flashes, to possess an appearance too slightly differing from that of a revolving light, to admit of its being safely adopted in situations where revolving lights are near. The trial which I made at the Little Ross, in the Solway Frith, of producing, by means of lenses, a light flashing once in five seconds of time, although successful so far as mere distinction is concerned, has several practical defects, arising from the shortness of the duration of the flashes, compared with the powerful effect of the fixed part of the apparatus, which I consider sufficient to prevent its adoption in future, especially considering that a much more marked appearance can be produced, by means of reflectors, as has been done at the Buchanness in Aberdeenshire, and the Rhinns of Islay in Argyllshire. Coloured media have never, so far as I know, been applied to Dioptric apparatus, except in the case of the Maplin Light at the mouth of the Thames, and Cromarty Point Light at the entrance to the Cromarty Frith, and in both instances successfully. The enormous loss of light, however, amounting to no less than 0·80 of the whole incident rays, forms a great bar to the adoption of colour as a distinction; and any means which couldtend to lessen that absorption, and at the same time produce the characteristic appearance, would be most valuable. I have tried some glasses of a pink tinge, prepared by M.Letourneauof Paris, whose absorption does not exceed 0·57 of the incident rays; but the appearance of the light, at a distance, is much less marked than that produced by the glasses used in Britain.[77]Such deficiency of characteristic colour might lead to serious consequences, as the transmission of white rays, through a hazy atmosphere, too often produces, by absorption, a reddish tinge of the light, for which the less marked appearance given by the paler media might be easily mistaken. This colouring power of absorption is so well known, that red lights are seldom used except in direct contrast with white ones; but, on a coast so thickly studded with Lighthouses as that of Britain, the number of distinctions is insufficient to supply all our wants, so that we are sometimes reluctantly compelled to adopt asingle red lightin some situation of lesser importance, or which, from some local circumstances and the appearance of the lights which must be seen by the mariner before passing it, is not likely to be mistaken. The great loss of light by coloured media causes the red beam, in a revolving light, to be seen at a shorter distance than the white; and it is conceivable that, in certain circumstances, this might lead the mariner to mistake ared and white lightfor awhitelight revolving at half the velocity. Such a mistake might perhaps prove dangerous; but the lights are generally so situated that there is ample time for the mariner, after first discovering the red light, and thus correcting any mistake, to shape his course accordingly. All other coloured media exceptredhave been found useless as distinctions for any lights of extensive range, and fail to be efficient, owing to the necessity of absorbing almost all the light before a marked appearance can be obtained. In a few pier or ferry lights, green and blue media have been tried, and found available at the distance of a few cables’ lengths.

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