(Continued from page 725.) OF all the substances which the genius of man has enabled him to extract from the crude matter of the earth, and to appropriate to his own use, there is perhaps none so beautiful as glass. Whether we look at the vessels of thousand shapes which ornament our festive boards and administer to our daily comfort, or at the stately mirror, or the noble modern window-pane, so pellucid as almost to deceive us as to its reality ; or whether we contemplate the wondrous lens, which enables us to shorten space, exposes to our view the otherwise invisible stars, and brings within our ken the microscopic world, we think there is nothing within the whole range of human manufactures that does not, in point of beauty, pale by comparison with it.

Amongst the numberless useful purposes to which this ductile, adaptable body has been appropriated, one of the most beautiful and valuable has been the illumination of the coasts of the sea.

In our last part of this Article, we stated that rays of light emanating from any luminous body could be collected and thrown in any required direction, either by reflection or refraction, and that the former mode, which we then described, was termed the catoptric system.

We have now to give some account of the latter and later mode, which, in contradistinction to the other, is called the dioptric system, catoptrics being that part of optics which treats of rays of light after being reflected, and dioptrics of refracted rays.

Until a comparatively recent period the use of lenses was confined to optiqal instruments in the usually understood sense of the term, such as telescopes, microscopes, eyeglasses, and spectacles, and to burning-glasses.

It may be thought strange that it did not occur to some opticians, at an earlier period, to apply them to lighthouse illumination; but it will appear less so when it is remembered that with the increase of size in a lens the thickness of the glass increases so greatly that the rays of light in passing through it become much obstructed, which would have been still more the case when the manufacture of that material was in a much more imperfect state than now,.and when, indeed, lenses of sufficient sizes could not have been manufactured free from flaws.

The first person to whom it occurred to get over this difficulty was the celebrated Buffon, who, to prevent the great absorption of light in a lens of large dimensions, proposed to grind out of a solid piece of glass a lens in steps or concentric zones, and burning- glasses were so constructed by the Abbe Rochon about the year 1780. Great difficulties, however, attended this process, and it was not followed up.

The merit of first proposing the building these lenses in separate pieces, which was the next step in advance, is due to Condorcet, who, in his "• Eloge de Buffon,' published in 1773, in remarking on Buflbn's lenses, suggests that they might be so constructed.

To our countryman, Sir David Brewster, however, is undoubtedly due the credit of first proposing, in the year 1811, the construction of lenses composed of several separate zones, and those zones of separate pieces, which he termed polyzonal lenses; and of first proposing, in 1815 or 1816, the application of such lenses to lighthouse illumination.

. , Lastly, in the year 1822, the celebrated French optician, M. Fresnel, who was unacquainted with Sir David's invention, proposed the very same thing, and was at once employed by the French Government to introduce the new lenses into all the lighthouses on the coasts of France. This mode of illumination has since, in consequence, been generally denominated the ' French system." However, although we believe that Sir David Brewster has never had full justice done to him for his repeated and persevering, although unavailing, efforts to introduce this dioptric system into British lighthouses, we must leave the claims of rival inventors to be settled in other works, and devote our limited space rather to what will chiefly interest our readers—a plain description of the valuable and beautiful system itself, many of the improvements and details of which were first practically carried out by the distinguished Frenchman above named.

M. Fresnel, indeed, appears to have been the first to have actually constructed an annular lens, and, in conjunction with MM. Arago and Mathieu, placed a powerful light in its focus, thus applying it to the practical purposes of a sea-light; and, accordingly, in the year 1823, a fine example of the apparatus, as arranged by him, was placed in the noblest existing light tower, viz., that of Cordouan, at the mouth of the Gironde.

From its complete success, it was made the basis for the illumination of the coasts of France, which, under the law of 1825, was then systematically arranged.

The dioptric system is dependent on the refractive properties of glass. When a ray of light passes obliquely from one transparent medium into another of different density, as from air into water or glass, it pursues a different direction at the common surface of the two planes, as may be observed by placing one end of a stick in a slanting or oblique direction under water, when, as is known to every one, it appears bent upwards, as if broken at the point in contact with the water's surface.

The same occurs to a ray of light in its passage through glass, and when it issues from the opposite surface to that at which it entered, its direction is again changed.

This change of direction, or the angle which the two directions make, varies with the density of the glass, and that which it makes with a normal or perpendicular is called the " index " of refraction. If the two surfaces of the glass are parallel, the issuing ray is in the same direction as the entering or incident ray; whilst, on the other hand, if the opposite surfaces of the glass are not parallel, the issuing ray will follow another course, and the light will pass in the three directions.

By varying the form and position of a lens, it is therefore evident that the rays of light emanating from any luminous body may be diverted from their first direction, and conveyed, within certain limits, in any direction required.

As the object to be attained by all seacoast lights, whether catoptric or dioptric, is to show a brilliant light to vessels floating on the sea, a band or belt of light, as it were, following the horizon, and having its upper and lower margin only a short distance above and below it, is all that is required.

As with the reflector, so therefore also with the refractive lights, all the otherwise wasted rays that would pass upwards into space, unseen by mortal eye, or downwards to the nearer surface of the sea or ground, have to be diverted from their onward course, and made to converge within the bounds of the horizon-belt to direct the mariner on his way.

To effect this object in a dioptric light a plano-convex form of lens is chosen, as being more easily worked, and for the purpose of more readily correcting the aberration for sphericity. Other optical qualities are disregarded, such as the chromatic aberration, as the light to be dealt with is not a minute point. As the flame is of considerable dimensions, its area neutralizes these difficulties.

The plano-convex lens then is applied with the convex side away from the focal lamp, and the incident rays from that focus are refracted in the body of the lens, and finally pass out horizontally as required.

The lenticular apparatus may be thus described:—It consists of a central and powerful lamp, of course emitting luminous beams in every direction. Around this is placed an arrangement of glass lenses, so formed as to refract these beams into parallel rays in the required directions.

The use of the glass lens is thus to bend the rays which fall on and emerge from its two surfaces. The action of the bull's-eye lantern, in sending forth the rays in one direction, will explain this principle. As the normal figure of the lens is that to which its powers are due, the annular or polyzonal lens must be considered as snch a complete lens with the unnecessary portions cut away.

One great advantage in the decomposition rig. i.

Diagram illustrative of the principle of the polyzonal lens. A B C la a gectton of an ordinary plano-convex lens whose focus is at F. As the great thickness of the central portion abstracts much of the light in its passage, the convex surface may be supposed to be cut into circular zones, whose section is as the shaded part of the diagram; and these sections being all placed in one plane, as A" B' C", the latter will have all the optical properties of the former, because the two surfaces are still of the same relative figure.

of the original lens, as shown in Fig. 1, is that of diminishing its weight very considerably, and also the greater certainty of the more uniform density of the material from which it is made.

The lens adapted for a first-order apparatus is of 36-22 inches focal length—has an area of about 1,300 square inches— weighs about 1 cwt., and costs 60Z.

This lens is only adapted to a revolving light. Eight of them are built into an octangular prism, M, B, N, C, Fig. 2, around the focal lamp F, and this made to revolve by regulated machinery. The effect to a distant observer is a brilliant flash, of the size of the lens, each time that one passes before his line of vision. The brilliancy of this flash, according to some calculations, is equal to that of 3,000 unassisted Argand flames. By other estimates it is made somewhat lower.

This portion of the apparatus 'only embraces an angle of about 46°, or rather more than one-fourth of the entire light.

The rest is economised on a different principle, as will be presently shown.

For a fixed dioptric light, a different modification of the same principle is used.

Instead of the central portion being made up of annular lenses, it is a cylindrical belt, of the section C, D, E, Fig. 3, similar to that of the lens, made to revolve around the focus F in a perpendicular direction. The effect of this central belt E E is, not to distribute the light into eight beams, but equally all round the compass—that is, from whatever direction it is viewed a bright band of light, of the breadth of the flame F from the lamp L, is visible from top to the bottom of the central belt.

View of a first-order revolving Dioptric Light with upper and lower reflecting Zones.

In the first instance this apparatus was made a polygon of 32 sides, but in 1836 Messrs. Cookson made one entire, which was the greatest step then achieved.

In the separate panels of which the cen- tral belt is made up, the sides are not perpendicular, but diagonal, M, N, Fig. 3, forming them into rhomboids, in order that the metal framework which holds them together should not obstruct the band of light throughout their whole length in anyone direction.

Fig. 3.

View of a first-order./;*«! Dioptric Ligbt, will) upper and lower refrading Zones.

For the remainder of the light, which passes over and under the central zone, different means have been applied to economize it. In the first apparatus, the revolving light of Cordouan, and in similar apparatus at the Skerryvore, a complicated arrangement of eight smaller lenses are placed over the principal lenses in a conical form.

These throw the light diagonally upward, and it is then received on to eight long plane mirrors, diverging outwards and upwards from the centre of the apparatus, which reflect the bright images of these eight smaller lenses into a horizontal direction, adding their power to that of the central lenses. . This form of apparatus, though beautiful and effective—and another later one—are, however, being superseded by a much more beautiful and effective plan for these supplementary zones. The first on a large scale were the lower zones of the Skerryvore light, by Alan Stevenson, since which they have become universal.

These are totally-reflecting-cato-dioptric prismatic zones—a long array of words, which, however, are expressive; the particular action of which may be explained by a very familiar experiment.

Place a stick of sealing-wax, a pencil, or any other substance, in a sloping direction from you in a tumbler of water. Raise the tumbler above the level of the eye, until, at a certain angle, you will see the image of the sealing-wax, &c., totally reflected under the upper surface of the water.

Importing the principle thus demonstrated into the lighthouse service, these zones act as may be thus explained. In Fig. 4 representing a section of a zone, A, D, C, which is so placed in regard to the focus F that a ray falling from it at/'will be so refracted on to the side D A at/1, that, instead of passing out, it will be totally reflected (as in the tumbler of water) at that point of incidence f", into the direction f", and finally pass out in the required direction. This angle fff" is less than 41° 49', as with more than this it would not be reflected, but pass out upwards.

Kg. *.

In a first-order light there are eighteen of these zones above the central belt, and eight below it; the entire apparatus forming a most beautiful object 10 feet high and 6 feet in diameter, costing, for the optical portion only, about 1,3001. The public were familiarized for the first time with it at the Exhibition of 1851, and the much more beautiful series shown in the nave of that of 1862, especially that by the Messrs. Chance, gained universal admiration.

The dioptric apparatus is divided by the French into six orders or sizes, from the powerful first order, 10 ft. in height, to the small harbour lens of 114 inches in diameter.

A still further improvement has been made by Mr. Thomas Stephenson, applicable to all lights which only require to be visible from a portion of the line of the horizon, its object being to utilize those rays of light which would otherwise be thrown behind the light or in the opposite direction to that portion of the horizon requiring to be illuminated.

This he has effected by cutting off the part of the ordinary reflector behind the focus, and substituting a hemispherical reflector behind the flame, and in front of it a lens with three diacatoptric rings. The action of this spherical reflector is to return all the rays impinged on it back through the flame, and thus on to the posterior sides of the lens and diacatoptric rings; whence it follows that all the rays which emerge from the lens, &c., will be horizontal, and the remainder—those which impinge on the parabolical—will also be reflected in the same direction.

This arrangement has been termed the holophotal system, from two Greek words, signifying " whole light." Having described this truly scientific system, by which, through the instrumentality of a beautiful material, we are enabled to bend and direct the rays of light at our will, we proceed to give some account of the light itself, which is special in its character.

Fresnel, on completing his invention, immediately perceived the necessity of combining with the dioptric instruments a burner capable of producing a large volume of flame; and the rapidity with which he matured his notions on this subject, and at once produced an instrument admirably adapted for the end he had in view, affords one of the many proofs of that happy union of practical with theoretical talent for which he was so distinguished. The lamp designed by him has four concentric burners, which are defended from the action of the excessive heat produced by their united flames by means of a superabundant supply of oil, which is thrown up from the cistern below by a clock-work movement, and constantly overflows the wicks, as in the mechanical lamp of Carcel. A very tall chimney is found to be necessary, in order to supply fresh currents of air to each wick with sufficient rapidity to support the combustion.

The carbonization of the wicks, however, is by no means so rapid as might be expected, and it is even found that, after they have suffered a good deal, the flame is not sensibly diminished, as the great heat evolved from the mass of flame promotes the rising of the oil in the cotton. So perfect, indeed, is the action of this great lamp, that it has been known to burn for upwards of twelve hours without being snuffed or even having the wicks raised.

The annexed diagrams will give a more perfect idea of the nature of the concentric burner than can easily be conveyed by words alone.

Fig. 5 shows a plan of a burner of four concentric wicks. The intervals which separate the wicks from each other, and allow the currents of air to pass, diminish in .width a little as they recede from the centre.

Fig. 6.

Fig. 6 shows a section of this burner.

C, C', C", C'" are the rack handles for raising or depressing each wick. A B is the horizontal duct which leads the oil to the four wicks; L L L are small plates of tin by which the burners are soldered to each other, and which are so placed as not to hinder the free passage of the air; P is a clamping screw which keeps at the proper height the gallery RR, which carries the chimney. Fig. 7 shows the burner with its glass chimney and damper. E is the glass chimney, F is a sheet-iron cylinder, Fig. 1.

o-s-0 ill which serves to give it a greater length, and has a small damper, D, capable of being turned by a handle for regulating the supply of air, and B is the pipe which supplies the oil to the wicks. The chief risk in using this lamp arises from the leather valves, that force the oil by a clockwork movement, being occasionally liable to derangement; and some of the lights on the French coast, and more especially the Cordouan, have been extinguished for a few minutes by the failure of the lamp—an accident which has never, and scarcely can happen with the fountain lamps which illuminate the reflectors. To prevent the occurrence of such accidents, and to render their consequences less serious, various precautions have been resorted to.

Amongst others, an alarum is attached to the lamp, consisting of a small cup, pierced in the bottom, which receives part of the overflowing oil from the wicks, and is capable, when ftdl, 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 running out at the bottom, the equilibrium of the lever is destroyed, and in falling it disengages a spring, which rings a bell sufficiently loud to waken the keeper should he chance to be asleep. There is another precaution of more importance, which consists in having always at hand in the light-room a spare lamp, trimmed and adjusted to the height fov the focus, which may be substituted for the other in case of accident. It, however, takes about twenty minutes from the time of applying the light to the wicks, to bring the flame to its full strength, which, in order to produce its best effect, should stand at the height of nearly four inches.

Such is a brief description of the dioptric system of sea-coast lights, the younger, and certainly not the least beautiful of the two sisters which, like the wise virgins of the parable, stand ever ready at their post of duty, with their lamps trimmed and their lights burning.

Lastly, we have to notice the fuel employed.

Up to the year 1846 all the British and Irish lights were obtained from the best sperm oil; since which period, however, colza oil only has been used, as it had previously been in the French lights.

Colza oil is made from the seed of the colza or colzut, a species of wild cabbage now extensively cultivated for the purpose in Normandy. It not only has the advantage of being cheaper than sperm oil, but is superior to it in quality for illuminating purposes, yielding a somewhat brighter and steadier light, and burning, with a thick wick, for no less than seventeen hours without requiring any cooling of the wick or adjustment of the damper.

The quantity of oil burnt annually in the great Fresnel lamp, of four wicks, is about 750 gallons.

Gas, with but few exceptions, in the neighbourhood of towns, has not been used for lighthouse illumination, as in. the majority of cases it could not be obtained.

It is, however, very suitable for the purpose both from its brightness and the facility it affords of producing a flame of any dimensions.

The attempt has more recently been made to adapt to lighthouse illumination the magneto- electric light, which far exceeds in brilliancy any light obtained by ordinary combustion, and Professor T. H. Holmes has produced a magneto-electric machine which produces the necessary continuous light of great brilliancy. The.lighthouse at Dungeness has been now for some time illuminated by Mr. Holmes' electric light.

It may at present be considered as an experimental light only; but it may be reasonably hoped and expected that it will be so perfected as to be available for coast illumination generally, when it will form an invaluable addition to the present system; especially as from its dissimilarity to all other lights it would afford the means for placing such lights as starting points in prominent positions, at sufficiently distant intervals, and so to decrease the possibility of one light being mistaken for another on those parts of the coast which are necessarily thickly studded with them. y- There remains but the concluding division of the subject, viz., the position in which the coast lights are, or should be, placed, and some consideration as to their distinctive character, which we must leave to our next Number.