530
[Class XXIV.
Mr. Ross, the celebrated optician of London , was thefirst to call attention to a defect, which may he detectedby polarization of light ; and there is no doubt but thatmany failures, which had occurred previously, were to beattributed t) the absence of this searching test. A glass,exceeding only the small diameter of 6 inches, undergoesthe annealiLg process with difficulty, and is liable to coolat the surface more rapidly, than in the interior; andthis tendency increases with the size, which rentiers theproduction >f a disc of 29 incites a very remarkable work.
In this section are comprised the optical glasses ofM. Maes, of Clichy, which have been distinguished bythe award of a Couueil Medal (see p. 532), and likewisethe annular lenses and cylindric refractors, which areapplied in the construction of lighthouses, according tothe principle introduced by Augustin Fresnel . Bufi’on inthe last century, and Sir David Brewster in our owntimes, had recognized the advantage resulting from thearrangement of lenses in separate pieces, having a commonfocus. Fresnel, without being aware of the exertionsmade by Sir David on the subject, submitted, in 1S22, tothe French Commission on Lighthouses, a plan for sub-stituting, along the coast, lights on this principle, andintroducing the refracting apparatus instead of the metallicreflectors then in use.
In 1S34, Mr. Alan Stevenson was sent to France tocompare the lenticular apparatus with the paraboloidalmirrors in use in our lighthouses, and to report on thecomparative merits of the dioptric and catoptric systems.The result has been the gradual introduction of lights onFresnel’s principle into our lighthouses, under the direc-tion of the Commissioners of Northern Lights, and Mr.Stevenson ’s superintendence. The Commissioners haveexhibited in the North-western Gallery some small lights(we presume) as models of those in course of adoption.
Two sets of lights Ixave likewise been constructed on alarger scale, of which one is exhibited by Messrs. Chance,another by Mr. Wilkins, of London . 'The latter is anapparatus containing the fixed light, varied with flashes,invented by Fresnel. The glass of which it is constructedwas made at St. Gobain , and wrought in Paris byM. Letouriu-au. It is white, and appears to be of goodquality; and if it is found not to be liable to humidity,the colour is certainly much in its favour.
The apparatus contributed to the Exhibition by Messrs,Chance was constructed at their manufactory, under thesuperintendence of M. Tabouret, formerly employed inthe department of the Pouts et Chaussees in France ,who, after thirty years of practice under that Board, hadaccepted the iuvitation of Messrs. Chance to direct themanufacture of the dioptric lighthouse apparatus, whichthey were desirous of establishing at their works. Theyhave exhibited a dioptric apparatus of the first order, withrevolving lenses and eatadioptric zones, constructed onthe principle of Fresnel. The upper and lower parts ofthe apparatus consist of a series of prismatic rings, eachof which reflects, at the internal surface of its base, theincident rays of light. The advantage of this mode ofreflection over the ordinary system of opaque reflectorsconsists in its saving a considerable loss of light, andbeing less liable to imperfection of surface. The middleportion of the apparatus is refracting, and produces by itsrevolution a succession of flashes or blazes of light, whichmay enable the mariner to distinguish any particularlighthouse. The revolving part consists of eight annularlenses. Each of these lenses is composed of a number ofconcentric rings round a central lens, so as to produce allthe refractive effect of a single solid lens of correspondingdimensions, but with less loss of light. A more perfectoptical figure is moreover given to these compound lenses jthan to a single piece of glass, spherical aberration being ■iu some measure corrected, and lenses of larger size may ,thus be formed than could otherwise be practicable. Theglass used in the apparatus now in the course of adoptionin the lighthouses on the coast of Scotland is made inFrance , and it lias been alleged that it is liable to attract
quality: and its merit has been so satisfactorily established ,as to justify the Jury of Class X. in recommending that aCouncil Medal be awarded to the manufacturers.
moisture or “sweat,” and consequently to lose a portionof its brightness. But the French manufacturers d-nythe charge; and when the judgment and care manifestedin the execution of these important works are taken intoconsideration, a strong presumption is afforded that theCommissioners and Mr. Stevenson as yet find the French glass the best adapted for the purpose. But, supposingthe contrary opinion to be correct, it only opens to theBritish manufacturer a fair field for competition, in wliichwe have no doubt of his being successful. Messrs. Chancehave acted upon this principle: they have made a firstexperiment with the view of promoting this branch ofthe manufacture of glass in England; but in order toavoid the defect of sweating, they have, perhaps, goneinto the other extreme, and though the glass of theirlights is good in quality, the measures taken to expel thesalts in its composition, which render it liable to thisobjection, have affected the colour, and given it a greentint, which is considered by competent judges to beobjectionable.
The question of lighthouses does not properlv comewithin the scope of this Jury; but having been calledupon to advert to the apparatus in the Exhibition, ofwhich the glass forms so material a part, we cannot quitthe subject without briefly noticing some of the pro-gressive improvements widely have taken place in thisdepartment of the public service. The first, or mostmagnificent work in modern times was the Tour deCorduan, founded in 1584 oil an extensive reef at themouth of the Garonne , but not finished till 1610. Woodand afterwards coal afforded the first light. In 1780,M. Lenoir was employed to substitute paraboloidal re-flectors and lamps, and in 1S22 the dioptric apparatus ofFresnel was introduced.
The Eddystone, 9.J miles from the Ram’s Head, on thecoast of Cornwall , was erected of timber by Winstanleyin 1096-98, and washed away in 1703. It was rebuilt byRudyard in 1706, and destroyed by fire in 175.). Thepresent edifice was erected by Smeaton in 1757-59.'Fallow candles were used in the first instance for thelights. In 1807 argand burners and paraboloidal re-flectors of silvered copper were substituted.
The Bell Rock Lighthouse, commanding the line ofapproach to the Firths of Forth and Tay, constructed ofstone by Mr. Robert Stevenson in 1807-10, is too wellknown, and reflects too much honour on his name, torequire any further notice.
The most remarkable work on the coast of Ireland isthat of Carlingford, near Cranfield Point, erected in1830.
The Skerryvore Rocks, about twelve miles south-westof Tyree, off the coast of Argyllshire, lying in the trackof the shipping of Liverpool and of the Clyde, had longbeen regarded with dread by the mariners frequentingthose seas. The extreme difficulty of the position, ex-posed to the unbroken force of the Atlantic Ocean , hadalone deterred the Commissioners of Northern Lightsfrom the attempt to place a light upon this dangerousspot; but in 1834 they caused the reef to be surveyed,and in 1838, Mr. Alan Stevenson , their engineer , inherit-ing his father’s energy and scientific skill, commencedhis operations on a site from which “ nothing could beseen for miles around but white foaming breakers, andnothing could be heard but howling winds and the lash-ing of the waves.” His design was an adaptation ofSmcaton’s tower of the Eddystone to the peculiar situa-tion—a circumstance with wliich he had to contend. Heestablished a circular base 42 feet iu diameter, rising ina solid mass of gneiss or granite, but diminishing indiameter to the height of 26 feet, and presenting an even,concave surface all around to the action of the waves.Immediately above this level the walls are 9*58 feetthick, diminishing in thickness as the tower rises to itshighest elevation, where the walls are reduced to 2 feetin thickness, and the diameter to 16 feet. The tower isbuilt of granite from the islands of Tyree and Mull, amdits height from the base is 138 feet 6 inches. In theintervals left by the thickness of the walls are the stairs,a space for the necessary supply of stores, and a notuncomfortable habitation for the three attendants. The