320
[United
horizontal direction. The object of this part is to turn touseful account, by prolonging the duration of the flash,that part of the light which would otherwise escape intothe atmosphere above the main lenses. This is effected bygiving to the upper lenses a slight horizontal divergencefrom the vertical plane of the principal lenses. Below, arefive tiers of totally reflecting prisms, which intercept thelight that passes below the great lenses, and by meansof two reflections and an intermediate reflection projectthem in the shape of a flat ring to the horizon.
Fixed dioptric apparatus of the first order (same asthat at Isle of May, with various improvements). Theprincipal part consists of a cylindric belt of glass, whichsurrounds the flame in the centre, and by its actionrefracts the light in a vertical direction upwards and down-ward so as to be parallel with the focal plane of thesystem. In this way it throws out a flat ring of lightequally intense in every direction. To near observers,this action presents a narrow vertical band of lightdepending for its breadth on the extent of the horizontalangle embraced by the eye. This arrangement, therefore,fulfils all the conditions of a fixed light, and surpasses ineffect any arrangement of parabolic reflectors. In orderto save the light which would be lost by passing aboveand below the cylindric belt, curved mirrors, with theircommon focus in the lamp, were formerly used; but onthe suggestion of the present engineer, the adaptation ofcatadioptric zones to this part of the apparatus was, aftermuch labour, successfully carried out. These zones aretriangular, and act by total reflection; the inner facerefracting , the second totally reflecting , and the third, orouter face, a second time refracting , so as to cause thelight to emerge horizontally. The apparatus has receivedmany smaller changes by the introduction of a new modeof grouping the various parts of the framework, by whichthe passage of the light is less obscured in every azimuth.During the last four years, these improvements have beenintroduced into the lighthouses in Scotland .
[The dioptric system of lights was invented by M.Fresnel a.d. 1819.—S. C-]
Model of the Bell Rock Lighthouse, lighted in 1811.This lighthouse was executed according to the design, andunder the superintendence, of the late Robert Stevenson ,Esq., F.R.S.E. , Engineer to the Commissioners of theNorthern Lighthouses. The model shows the progressof the work; the temporary barrack-house of timber;the railways, stone lighters, and cranes used in its con-struction ; and on the pane at one end of the case, thecourses of masonry, with their dovetails and joggles.Cost 61,331/. 95. 2d. The bird on the top of the modelwas carried against the lantern in a gale, broke a pane ofglass, injured one of the reflectors, and fell dead on thefloor of the lightroom.
Model of the Skerryvore Lighthouse, on the small scaleas No. 3. The light was exhibited in 1844. The light-house was executed according to the design and under thesuperintendence of Mr. Alan Stevenson , LL.B. F.R.S.E.,the present engineer . Cost 86,977/.
Model of balance crane, designed by the late Mr. RobertStevenson , and used by him in the erection of the BellRock Lighthouse, for raising and setting the stones atany distance from the centre of the tower, in which thestone and the counterpoise balance each other by beingcontinually kept at an equal distance from the shaft ofthe crane.
Model of the apparatus of an intermittent light. De-signed by the late Mr. Robert Stevenson , and introducedby him at the lights of Tarbertness, Barrahead, and Mullof Galloway, on the coast of Scotland . This apparatuseclipses the light suddenly for half a minute of time, andas suddenly reveals it to view, when it continues in sightfor two and a half minutes, and is again suddenly eclipsed;thus producing its whole effect in three minutes of time.The suddenness of its eclipse and reappearance, and theinequality of the periods of light and darkness, form amarked contrast to the gradual waxing and waning whichprecede the brightest phase and dark periods of a revolving
light, all the phenomena of which recur at equal intervalsof time.
Model of a lighthouse lantern, on the diagonal arrange*ment. Designed by Mr. Alan Stevenson , for the purposeof avoiding the unequal distribution of the light to whichthe old lanterns were liable from the interception of raysin certain azimuths by means of the vertical astragals.Great rigidity and strength are also insured by thisarrangement, and the triangular form of the panes rendersthem less liable to be broken by the force of impingingbodies.
Mechanical lamp of four wicks, in which the oil is keptcontinually overflowing by the means of pumps, whichraise it from the cistern below; the rapid carbonization ofthe wicks which would be caused by the great heat is thusavoided. The flames of the lamp reach their best effectin three hours after lighting, i. e. f after the whole of theoil in the cistern, by passing and repassing over the wicksrepeatedly, has reached its maximum temperature. Afterthis the lamp often burns 14 hours without sensiblediminution of the light, and then rapidly falls. The lightvaries from 16 to 20 times that of the Argand flame of aninch in diameter ; and the quantity of oil consumed by itis greater nearly in the same proportion.
Holophotal arrangement of lighthouse apparatus, pre-pared by Mr. Thomas Stevenson , C.E., in which thewhole sphere of light is brought under instrumental actionthus:—The posterior hemisphere of light is reflected bythe hemispherical mirror (and in some cases by reflectinghemispheres of glass), and passes with the usual loss byreflection again through the flame, where it is paral-lelized, partly by refraction at the lens, and partly by theparabolic mirror which is truncated in the parameter.The anterior hemisphere of light is also parallelized*partly by the action of the lens, and partly by theparabolic mirror. In this manner there is no loss of theposterior portion as in the lens, nor of the anterior portionas in the mirror.
100 Stevenson, Thomas, F.R.S.E. , 84 George Street ,Edinburgh —Inventor.
Revolving light, with axial rotation, by which one-haRthe number of reflectors, and one-half the quantity of 01 *’are designed to be saved. Intended for illuminating anfarc of not more than 180 degrees. The intervals of time °illumination are equal within the whole of the illumi»at earc, instead of unequal as in the reciprocating light. Threflectors are also of a new form, consisting of parabohstrips of different focal distances.
Ordinary parabolic reflector, rendered holophotal C*’ 11 ?e entire light is parallelized) bv a portion of a catam'
the
ab°li c
optric annular lens. The back part of the para° ^conoid is cut off, and a portion of a spherical mirror s°stituted, so as to send the rays again through the fl alT [ 'All the light intercepted by the annular lens is lost in iordinary reflector.
Holophotal catadioptric annular lens apparatus y l T,finished). This is a combination of a hemispherical n } irr () fand a lens with totally-reflecting zones; the peculiarly ^this arrangement is, that the catadioptric zones, instead.^transmitting the light in parallel horizontal plates, a s ^Fresnel’s apparatus, produce, as it were, an extension ^the lenticular or quaquaversal action of the central l e ^by assembling the light around its axis in the ford 1concentric hollow cylinders. then 1
(The above instruments belong to the Board of N°rtLights.)
[The early method of illuminating lighthouses ^coal or wood fires contained in “ chauffers.” The I s , ^Man light was of this kind until 1816. The firstimprovement was made by Argand, in 1784, who mya lamp with a circular wick, the flame being suppd ge
an external and internal current of air. To make ^
lamps more effective for lighthouse illumination,prevent the rays of light escaping on all sides, a 1 ^
was afterwards added: this threw the light f° lffa