244
HISTORY OF ENGINEERING.
Book I.
mirrors; and sometimes as many as eight are employed, mounted upon a frame, their axesbeing all parallel to each other, and so placed that the light reflected by them is formedinto one conical beam.
A revolving light ,was produced by at-taching the frame to a horizontal axis,made to turn round by the aid of ma-chinery.
A stationary light only requires the re-flectors to be placed round a circular frame,with their axes on the radii of the circle.
The illumination, however, is not equallyintense at all azimuths, but strongest in thedirection of the several axes, and weakestin that of the lines bisecting the severalangles formed by each pair of contiguousaxes.
Auguste Fresnel, of the Academy of Sciences at l’aris, advised that refractedlight should be generally introduced intothe lighthouses in France , having a planeconvex mirror with a focal distance of about8 feet, formed of crown glass, which wasthought less liable to stria.' than flint. Thefirst lens was polygonal, and consisted ofseveral pieces of glass, separately prepared,and united together, but afterwards sphe-rical lenses accurately ground supplied theirplace.
The divergence of a cone of light pro-jected by such a lens, is not more than5° 9', and is much less than that producedby the parabolical reflector.
The largest lens at the French light-houses projects a cone of light equal inintensity to eight mirrors of the best kind.
The luminous cone in one case is only thatportion of light which falls on the surfaceof the lens, and it might be imagined, thatthis could never equal in effect that pro-duced by the parabolic mirror; but thepolish of the latter not being perfect, agreat quantity of incidental light is lost,which is a chief reason in favour of the lens.
\\ hen retracted light is adapted to the revolving system, the frame which carries the lenseshas eight sides, to each of which one is attached ; the whole, however, are so arranged, thatall their axes are in the same horizontal plane, and meet in the common focus whore the.'amp is situated, forming one large octagonal prism.
To prevent any loss of light, a second frame is placed above the first, the sides of whichform the frustum of an octagonal pyramid, and incline fifty degrees; on each of these sidesis another lens, having its focus in the flame of the lamp. The rays which fall on theinclined lenses are retracted parallel to the axis of the lens, and are then reflected into thehorizontal direction, by plane mirrors, placed above the upper frame. Curved reflectorsare sometimes used, above the frames which contain the principal lenses.
Fresnel also substituted a very elegant contrivance for the upper lenses and mirror : a seriesof triangular prisms had their axes arranged in horizontal planes, and so adjusted that thelight falling on the face next the flame was thrown upon the back of the prism, where itwas totally reflected; and by a second refraction at the third side of the prism, it obtainedits horizontal direction.
For fixed lights of this description, a sufficient number of lenses are required to formwith them a cylinder, so that an equal diffusion of light should be spread over every partof the horizon. Some French lighthouses have a refracting apparatus consisting of a beltof thirty-two lenses, arranged polygonally.
Such a light is described by Fresnel in his memoir on the subject, published in the year1822, and his system is applied to most of the lighthouses on the French and Dutch coasts.
The Argand fountain lamp, with a burner an inch in diameter, tipped with silver, is madeuse of; and I*resnel invented one with a series of concentric burners; for lights of the firstclass, there were usually four, protected from the excessive heat by a superabundant
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