THE INDUSTRY OF ALL NATIONS.

an Argand burner, by closing the apertures over the drip-pan, through which theinterior current of air is supplied. The lamp instantly smokes terribly, while onopening the apertures again, the smoke ceases, and the flame falls and becomeswhite.

The oil used for these lamps was at first spermaceti oil, or olive oil. Latterly,in England, but especially in France, oil of Colza, obtained from a species of wildcabbage and similar to what is known here as rape-seed oil, has been extensivelyintroduced, and is preferred on account of giving a more intense brilliancy, steadierflame, and less charring of the wick, as well as on account of its greater cheapnessin the market than any other oil in use.

Other materials for illumination have been, it is true, proposed at varioustimes in the last twenty-five years, but none that appear, on a consideration of allthe circumstances, superior to oil. Thus the use of carburetted hydrogen gasdistilled from coal, rosin, or oil, such as is used for street-lighting, has been sug-gested ; but the greater risk of irregularity in the manufacture and supply, aswell as the inconvenience of feeding with it those lights that are intended to re-volve (at least on the reflecting system), have been judged to render it inexpedi-ent. The voltaic light of Mr. Gardner, produced by a current of electricitybetween two nicely adjusted charcoal points; the Drummond light, arising fromthe ignition of lime in the flame of an oxy-hydrogen blow-pipe; and the Budelight of Mr. Gurney, in which oxygen (instead of the dilution of that gas inatmospheric air), is furnished to support the combustion of oil, all afford a flameof great brilliancy and intensity, but are so comparatively complicated and uncer-tain as to be of disadvantageous, or at least doubtful application, in a systemwhose purpose requires every arrangement to be simple, uniform, and unfailing.

But whatever may be the material for producing the flame, it is manifestwithout any particular investigation of the laws of light, that of a mere nakedflame, a large part (probably 7-8 of the whole), will be diffused without servingany useful purpose. This useful purpose in the case of great sea-lights, is, to bevisible at as great a distance as possible; hence, with these, the rays or beams oflight require to be nearly horizontal. In smaller lights, such as for harbors,channels, &c., the purpose is, to be visible close at hand; for which a greaterdivergence or throwing down of the beams upon the surface of the water is requi-site. In either case it is obvious that the whole of the upper part of the flameabove its centre, from which the beams are virtually radiated, is useless, for itstrikes above its horizon where it could not possibly be seen, unless some auxiliarycontrivance be adopted for catching, as it were, those stray rays, and divertingthem in a proper direction.

As it happens in many instances that only a part of the horizon is seaward,and therefore needs the illumination, the earliest used of such auxiliaries would bea flat reflecting surface, like the plate of brass which, so late as the beginning ofthis century 7 , was to be seen on the landward side of the flame at some of theEnglish Light-Houses. But a flat surface -would soon be found inefficient, and,in point of fact, would not collect more than 5 or 6 per cent, of the otherwise lostlight. A spherical one would be better, and we may suppose followed next inimprovement, yet leaving much to be desired. Geometers had long known theproperties of another curved surface, which they term paraboloidal, in the capa-city which it has of transmitting in a direction parallel to its axis, all beams thatradiate on it from a particular point called its focus ; but the mechanicians wereeither not properly invoked, or else shrunk from the practical difficultiesof executing a reasonably correct surface of the form required. Small panes orfacets of looking-glass were tried, set in paraboloidal moulds of wood or plaster,but at last, stimulated by the improvement in the light of Argand’s lamp, thegenius of Borda, about 1784, triumphed over the obstacles, and caused the erec-tion in the Oordouan Tower of really paraboloidal metallic reflectors.

Since then, the immense advantage of this method has caused it to be adoptedevery where under various modifications, and as every system, to be known musthave a name, this from the Greek word expressive of its most remarkable feature,is designated as the Catoptric system.

It is clear, nevertheless, from the form of this curved surface (which is most likethe larger end of an egg-shell broken transversely about one-third of its length fromthat end, and with a luminous point placed about two-thirds of the depth inside),that the efficiency of the light within is laterally very much restricted by thesides of the reflector, beyond which the flame would be of course masked. In pointof fact, a single lamp and reflector is only brilliant over about 4 per cent., or theJjth of the horizon, to extend which arc of efficiency, it is of course necessaryto place the lamps and reflectors themselves in an arc, or where the whole hori-zon requires to be illuminated (as in great sea-lights situated off the mouthsof estuaries) in the circumference of a circle. In order to obtain the requi-site quantity of light, lamps so arranged are placed tier above tier, until foundsufficient.

Another resort was had by Borda, which, as affecting another point of greatimportance in the distinctive character of lights, is of immense interest, while itanswered also the immediate aim of being visible over the whole horizon—inmaking the frame that carried the lights revolve. In this arrangement, thelamps and reflectors are set, instead of in a circle, tier above tier on the sides of a

square or polygon; and, as they turn on a central vertical shaft, each set oflamps successively throws its light over every point of the horizon. Asthe rate of revolution is quite rapid, the intervals of greatest brilliancy atany given point are very short, not exceeding a few minutes.

In this -way, both because the impression on the eye of the observer from thefirst beam, for instance, is augmented by that from the quickly following secondbeam, and because the lamps can be more conveniently adjusted on a plane thanon a curved surface, revolving lights are, with the same number of lamps, burn-ing a uniform quantity of oil, virtually more luminous than if the frame werefixed. Besides, by altering the rate of revolution, the intervals between thegreatest and least brilliancy (the first occurring when the observer is directlyopposite the lamp-bearing face, and the last when the dark angle of the frame isin line with him), may be altered too, so as to give within certain limits quite amarked and distinctive character to the light in question..

Yarious other modes (some of them of extreme ingenuity, and among thesethat of Bordier Marcet, the pupil and successor of Argand) have been proposedfor illuminating an entire horizon at once, by cutting away the closed end of areflector, and thus retroverting the rays. But none of these appear to have com-manded an undoubted preference, and therefore need not be spoken of here, wherethe object is more to indicate the actual, than to speculate on the possible.

Perhaps one of the most efficient obstacles to the success of these contrivances,which with all their geometrical profundity are rather complicated and fatiguingto be considered, has been in the adoption of another system, proceeding uponan opposite principle; and which, instead of reflecting the rays by a polishedsurface, causes them to pass through glass, and to be, as it is called, refracted. Inthe nomenclature of this, as of the other system, the classic language of Greecehas been resorted to, and from the old word in that country signifying to bevisible through , it is termed the Dioptric System. And as in the progressiveimprovement towards economy of light, reflections of the wandering rays wereproduced by special contrivances, it has also seemed proper to designate it as acompound system by the title Oata-dioptrio or Dia-oatoptrio._ This alternativeepithet is, to be sure, used more accordiug to the taste of the person employingit, than according to any established rule.

This system, however entitled, is due to Augustin Fresnel, a Frenchman bybirth, but a cosmopolite by genius, whose name will ever be recorded among thehighest of those whose researches in pure science have been applied by them-selves to the vast practical benefit of mankind. And it was with the severelogic grown familiar to him in such researches, that he was enabled to sweepaside from the practical problem the vague crudities of those who had precededhim, and to go at once, unerringly and unfailingly to his well-defined and bene-ficent aim.

The principle of this dioptric system is easily intelligible to any one who hasever amused himself with a burning-glass, or sun-glass, or magnifier. In that,the rays from the sun, which from the great distance traversed may be assumedas being parallel, are bent from their rectilineal course both on entering and onleaving the glass, so as to be converged to a point or focus, which is brilliant andheating in proportion to the size of the glass, and therefore the number of parallelrays falling on it. Mow if we consider the condition reversed and the luminouspoint or light at the focus to be pre-existing, it is evident that the rays divergingfrom it towards the glass, will be bent in passing through, and must come outparallel on the other side. This geometers and opticians knew, and they alsoknew, before Fresnel, how to calculate the amount of bending or refractionwhich must take place in a piece of glass of a given convexity. So, a hundredyears ago, lenses were actually applied in several light-houses in England andIreland, but the practical conditions conformed so badly to the theoretical, thatthe implements became consumers instead of economizers of light. The princi-pal difficulty seems to have arisen from the great thickness at the centre given toa uniform lens cut from one piece. Buffon conceived the idea of cutting away agreat part of this superfluous thickness, and of cutting the lens in concentricechelons. The keenness of this conception was more than neutralized by themechanical difficulties of the execution, and except the two glasses of Rochon andOookson, no one has been bold enough to try the experiment again.

The idea of Buffon seems to have been that the lens, to work satisfactorily,must be of one homogeneous and continuous piece; and this seems to have possessed him, although he saw so clearly that breaking up the curved continuity ofsurface would not embarrass the result, and was therefore just upon the righttrack.

Oondorcet, who, in his capacity of Secretary to the French Academy ofSciences, pronounced the eloge of his illustrious fellow-member, and who there-fore had studied connectedly and dispassionately the progress of discovery inScience and Art, was not thus pre-occupied, and at once seized upon and hap-pily expressed the idea of building up a lens in separate pieces. But neitherhe nor Brewster, who, in 1811, spoke judiciously (as he always does) in relationto the same suggestion, followed it up; and it was reserved for Fresnel, in 1822,both to describe the theoretical principles, and to give the practical formulas forwhat he termed annular lenses. In this we hardly know which most to admire—■