Optical The object of using optical apparatus in a lighthouse is to direct, as far as possible, all the rays of light that proceed from the radiant, or source of light, so as to be visible only on the sea between the near sea and the sea horizon. In the Eddy stone lighthouse, up to the commencement of the 19th century, the lighting apparatus con sisted of a chandelier of tallow candles, though parabolic reflectors made of facets of silvered glass mirror, set in a mold of plaster of Paris, were introduced in 1768 and used in several lighthouses, the effect of these reflectors being to concentrate and throw seaward the rays of light from a flat-wick 'aim placed in the focus. At a later date reflectors composed of sheet copper, plated with silver and formed into a parabolic curve, were largely introduced and are still in use. These reflectors, where a fixed light was desired, were arranged in two tiers on a frame, as many as 26 being necessary to show a light of equal power all around the horizon, and in the case of flashing lights seven were grouped on each of four faces of a frame that was rotated by clockwork. This mode of lighting is termed cotoptric, or reflect ing system. The method of building lenses of separate rings or prisms of glass, so as to form a larger lens than could be obtained from a mere bull's eye formed of one piece of glass, was first adopted for lighthouses in 1822. For a fixed light the built-up lens was so arranged as to form a polygon with one burner in the focus, and for a flashing light annular lens panels were grouped round the one central burner and revolved by clockwork. In 1892 Charles A. Stevenson made the lenses spherical in form in the vertical plane, and in 1895, with great advantage of the power of the light, abol ished the old section of the elements and gave them an equiangular section. This equiangular profile also permits of the refracting elements being extended to 80 degrees of focal open ing, and indeed farther, without loss of effi ciency. The setting of lenses eccentrically has rendered possible a diminution in the diameter and consequent saving in cost of lantern and tower. Besides the fixed-light apparatus and the lens panels many other forms of prisms for various purposes have been introduced. Thomas Stevenson's catadioptric mirror, formed of totally-reflecting prisms, and subsequently improved by James I. Chance, is largely used; and vertical straight prisms, placed in front of either fixed light or holophotal panels, are used to deviate the light azimuthally over par ticular arcs where the light is required. The desire to increase the power of the lights in duced some lighthouse authorities to advocate the use of larger burners without increasing the focal distance of the apparatus, with the result that little advantage was gained, as most of the light was ex-focal. It was not, however, till 1885 that the first lens was constructed, with the result that when tested it was found to produce a beam twice as intense as that from the previous lenses with the same large burners in the foci. This hyper radiant lens is now largely used where great power is desired. See PLATE.
radiants used in the focus of the apparatus in foreign lighthouses are generally 1-, 2-, 3-, 4-, 5-, 6-, or even 10-wick cylindrical paraffin burners, though gas burners, incandescent burners and the electric light, both and incandescent, are also in use. The use of paraffin resulted in a large saving. The elec tric light is now generally used in the United States. The popular idea that the electric light is not so good in fog as oil or gas lights was confuted in 1885, when it was found that oil and gas were usually affected by atmospheric variations, that the electric arc light is absorbed more largely proportionately to its initial power by haze or fdg than oil or gas lights, but that in all weathers and at all distances its pene trative power was found superior to the gas and oil lights, and that all three are nearly equally affected by rain. These results are confirmed
by practical experience in our lighthouses.
To enable the sailor to distinguish one lighthouse from another, lights in proximity to one another are arranged to exhibit different characters. The characters in common use are: fixed light; flashing light, showing one flash at regular intervals of a few seconds; group flashing lights, showing two or more flashes in quick succession, followed by a longer period of darkness than that which separates the flashes; occulting lights, which show a fixed light and are eclipsed for a few seconds at regular intervals. Colored lights, red and green, are also used with any of the foregoing characters to produce further dis tinctions, but in general only to mark danger arcs, or in conjunction with a white flash,. as the tinted-glass shades interposed to produce the desired color seriously cut down the power of the light, and are not, unless of a very dark shade, easily distinguishable in foggy weath r from a white light.
To produce these various char acters the lenses are placed on a carriage which revolves on conical rollers, or is floated in a mercury trough, and is driven round by clock work actuated by a falling weight. The tend ency has been in recent years to drive the apparatus faster, so as to make the period phase of the light as short as possible. While this is a desirable object it involves at the same time shortening the duration of the flash on the eye of the sailor, to which there is ob viously a limit, if distinct vision is to be ob tained under practical conditions. A flash of about half a second in length is regarded as what should be aimed at. The light on the Navesink Highlands above Sandy Hook, N. J., is one of the largest of modern flash lights.
The apparatus is placed in a glazed lantern erected on the top of the light house tower. With the view of intercepting as little light as possible, the framing or sashes are made of as small sectional dimensions as is con sistent with strength, and are made either diagonal, with diamond-shaped flat panes, or helical with curved panes. The upper part of the lantern is made dome-shaped with a ven tilator to carry off the fumes from the lamp. The size of the lantern varies with that of the apparatus, the usual size for a first-class light being 12 feet in diameter and 10 feet height of glazing.
LighftWps.— In certain situations, such as on rocks where there is not sufficient room to get a large enough base for a tower, or on sandbanks where the sand is liable to shift, it is impossible, except at a prohibitive cost, to erect towers to carry the light. In such situa tions recourse is had to mooring in the vicinity a vessel which carries the light on a mast. The light is generally shown from a lantern formed round the mast, and the apparatus consists of parabolic reflectors or small dioptric apparatus. These light-vessels have a crew consisting of a master, mate and nine men.
Beacons and Buoys are used in situations where powerful lights, such as can be shown from lighthouses and lightships, are not neces sary, and where the extinction of the light would cause inconvenience and not disaster, but where some guidance is desirable, as, for ex ample, in narrow sounds, rivers and estuaries. Beacons are now frequently lighted with small dioptric apparatus, the illuminant being either compressed oil-gas stored in a receiver, in which case they need no attention for several weeks, or with oil-burners, in which case they must be trimmed every three days. Buoys are made of various shapes to denote on which side of them the safe channel lies. Thus, can shaped buoys, those with a flat top, are to be passed on the port hand, and conical-shaped buoys on the starboard hand when the ship is going up an estuary or with the flood-tide, and vice versa. Spherical buoys denote a mid dle danger which may be passed on either hand.