REFLECTOR FOR A LIGHT-HOUSE, is composed of a number of square plane glass mirrors, similar to those with which Archimedes is said to have set fire to the Roman fleet at the siege of Syracuse (See BURNING, Encycl.) Each of these mirrors is about an inch square; and they are all disposed close to each other in the concave of a parabolic segment, formed of stucco or any other proper bed. Stucco has been found to answer the purpose best; and is accordingly employed in all the reflectors of the light-houses erected by Mr Thomas Smith tinplate worker, Edinburgh, at the expence, and by the authority, of government. This ingenious and modest man seems to have conceived the idea of illuminating light-houses by means of lamps and reflectors instead of coal-fires, without knowing that something of the same kind had been long used in France: he has therefore all the merit of an inventor, and what he invented he has carried to a high degree of perfection.
His parabolic moulds are from three to five or six feet in diameter; and in the centre or apex of each is placed a long shallow lamp of tin plate, filled with whale oil. In each lamp are six cotton wicks, almost contiguous to each other, which are so disposed as to burn without trimming for about six hours. The light of these is reflected from each mirror spread over the concave surface, and is thus multiplied, as it were, by the number of mirrors. The stucco moulding is covered on the back with tin-plate, from which a tube, immediately over the lamp, proceeds to the roof of the
Reflector, light room, and serves as a funnel, through which the smoke escapes without sullying the faces of the mirrors.
The light-room is a cupola or lantern of from eight to twelve sides, composed entirely of glass, fixed in cast-iron frames or fashions, and roofed with copper. On circular benches passing round the inside of this lantern, at about eighteen inches from the glass frames, are placed the reflectors with their lamps, so as that the concave surfaces of two or three of the reflectors front every point of the compass, and throw a blaze of light in all directions. In the roof immediately over the centre of the room is a hole, through which pass all the funnels already mentioned, and which serves likewise to admit fresh air to the lamps. This light-room is firmly fixed on the top of a round tower so as to be immovable by the weather; and the number of the reflectors, and the height of the tower, are less or greater according as it is the intention that the light should be seen at a less or a greater distance.
A man judging from mere theory would be very apt to condemn light-houses of this kind; because the firmest building shakes in a violent storm, and because such shaking, he might think, would sometimes throw the whole rays of light into the air, and thus mislead the bewildered seaman. This opinion, we know, was actually entertained of them by one of the profoundest philosophers and most scientific mechanics of the age. Experience, however, has convinced him, as well as the public at large, that such apprehensions are groundless, and that light-houses with lamps and reflectors are, in every point of view, preferable to those with fires burning in the open air. They are supported at much less expense; their light is more brilliant, and seen at a greater distance, whilst it can never be obscured by smoke, or beaten down on the lee-side by a violent gust of wind; and what is perhaps of still greater importance, the reflectors with their lamps may be so variously placed, that, as Mr Smith observes, one light-house cannot be mistaken for another. If we add to all this, that the lamps do not stand in need of trimming so often as open fires require fuel, and that the light man is never exposed either to cold or to wet by attending to his duty, we must be convinced that light-houses with reflectors are much less liable to be neglected in stormy weather than those with open fires, and that this circumstance alone would be enough to give the former a preference, almost incalculable, over the latter.
It has been proposed to make the concave surface of the parabola one speculum of metal, instead of covering it over with a multitude of plain glass mirrors; or to diminish the size of each mirror, if they are to be retained in preference to the metallic speculum. To every man who has but dipped into the science of optics, it must be obvious, that either of these alterations would be wrong. The brightest metal does not reflect such a quantity of light as well foliated clear glass; and were the size of the mirrors to be diminished, the number of joinings would be increased, in each of which some light is lost, not merely in the seam, but from its being almost impossible to foliate glass perfectly at its edge.