LIGHTHOUSE ILLUMINATION, In the entoptric or reflecting system all of those rays proceeding from the focus of a parabolic mirror which fall upon its surface are reflected parallel to the axis so as to form a solid beam of light. When a series of such reflectors are arranged close to each other round a cylinder in a lighthouse, they illuminate constantly, though not with equal intensity, the whole horizon. As the property of the parabolic reflector is to collect the rays in cident upon its surface into one beam of parallel rays, it would he absolutely impossible, were the flame from which the rays proceeded a mathe matical point, to produce a light which would illuminate the whole of the horizon. unless there were an infinite number of reflectors. (See PARABOLA.) But as the radiant, instead of be ing a mathematical point, is a physical object, consisting of a flame of very notable size, the rays which come from the outer portion of the luminous cone proceed, after reflection, in such divergent directions as to render it practically possible to light up, though unequally, the whole horizon. The useful divergence produced in this way by a burner of one inch in diameter, with a focal distance of four inches, is in the horizontal plane about 14° 22'. The whole horizon may thus be illuminated by reflectors. If, for the purpose of distinction, it is desired to show a ?Training light, then several of those reflectors arc placed with their axes parallel to each other on each of the faces of a four-sided frame, which is made to revolve. in such a ease, the mariner sees a light only at those times when one of the faces of the frame is directed toward him, but at other tittles he is left in darkness. The rotation of the frame upon its axis thus his eye a succession of light and dark intervals, which enables him to distinguish it from the fixed light which is constantly in view in every azimuth. The distinction of a red light is pro duced by using a chimney of red instead of white glass for each burner.
The flashing or scintillating light, giving, by rapid revolutions of the flame. flashes once every five seconds, which is one of the most striking of all the distinctions, was first introduced by Robert Stevenson, the well-known lighthouse en gineer, in 1822. at Rhinns of Islay, in Argyle shire. The same engineer also introduced what has been called the intermittent light, by which a stationary frame with reflectors is in stantaneously eclipsed, and is again as suddenly revealed to view by the vertical movement of opaque cylinders in front of the reflectors. The intermittent is distinguished from the revolving light, which also appears and disappears succes sively to the view, by the suddenness of the eclipses and of the reappearances. whereas in all revolving lights there is a gradual waxing and waning of the light. Wilson introduced at Troon Ilarbor an intermittent light which was produced by a beautifully simple contrivance for suddenly lowering and raising a gas-tlame. Stevenson proposed an intermittent light of unequal periods by eausing unequal sectors of a spherical mirror to revolve between the flame and a fixed dioptric apparatus (such as that shown in Fig. 3). The power of the light is increased by the action of the spherical mirror, which also acts as a mask in the opposite azimuths.
Another method of bending the divergent rays proceeding from a lamp into such directions as shall be useful to the mariner is that of refrac tion, known as the dioptric system. If a flame he placed in the focus of a lens of the proper form, the diverging rays will be bent parallel to each other, so as to form a single solid beam of light. Fresnel was the first to propose and to
introduce lenticular action into lighthouse illumi nation, by the adoption of the annular or built lens, which had been suggested as a burning in strument by Iltiffon and Condorect. lie also. in eonjunetion with Arago and Alathien, used a large lamp having four concentric wicks. In or der to produce a revolving light on the lenticular or dioptrie system, a different arrangement was adopted from that already described for the eatoptric system. The large lamp was now made a fixture, and four or more annular lenses were fitted together, so as to form a frame of glass which surrounded the lamp. When this frame is made to revolve around the lamp. the mariner gets the full effect of the lens whenever its axis is pointed toward him and this full light fades gradually into darkness as the axis of the lens passes from him. In order to operate upon those rays of light which passed above the lens, a system of double optical agents was employed by Fresuel. These consisted of a pyramid of lenses with mirrors placed above at the proper angle for rendering the rays passing upward parallel to those which came from the annular lens. But Fresnel did not stop here, for, in order to make the lenticular system suitable for fixed as well as revolving lights, he designed a new optical agent, to which the name of cylin dric refractor was given. This consists of cylindrical lenses, which were the solids that would he generated were the middle vertical profile of an annular lens made to revolve around a vertical axis. The action of this in strument is obviously, while allowing the rays to spread naturally in the horizontal plane, to .suf fer refraction in the vertical plane. The effect of this instrument is, therefore, to show a light of equal intensity constantly all around the hori zon, and thus to form a better and more equal light than that which was formerly produced for fixed lights by parabolic reflection. It is obvious, however, from our description that the diverging rays which were not intercepted by this cylindric hoop. or those which would have passed upward and been uselessly expended in illuminating the clouds, or downward in uselesK ly illuminating the light-room floor. were lost to the mariner; and in order to render these effective Fresnel ultimately adopted the use of what has been called the internal or total reflec tion of glass: and here it is necessary to explain that one of the great advantages of internal reflection by glass over metal is the smaller quantity of light that is absorbed. It has been ascertained that there is a gain of nearly oni: fourth (.249) by employing glass prisms instead of metallic reflectors for lighthouse There were introduced above and below the cy lindric refracting hoop separate glass prisms of triangular seetion, the first surface of each of which refracted to a certain extent any ray of light which fell upon it, while the seeond surface was placed at such an angle as to reflect, by total reflections, the ray which had before been refracted by the first surface: and the last or outer surface produced another refraction. which made the rays finally pass 'ilia parallel with those refracted by the central cylindric hoop. The light falling above the cylindric hoop was this IT refractions and reflections bent down ward. and that falling below was bent upward. so as to be made horizontal and parallel with that proceeding from the refracting hoop. Fig.