Page:The New International Encyclopædia 1st ed. v. 12.djvu/283

* LIGHTHOUSE. 257 LIGHTHOUSE. 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 phine about 14° 22'. The whole horizon may thus be illuminated by reflectors. If. for the jiurjiosc of distinction, it is desired to show a rcvijlving 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 case, the mariner sees a light only at those times when one of the faces of the frame is directed toward him, but at other times he is left in darkness. The rotation of the frame upon its axis thus produces to his eye a succession of light and dark intervals, which enables him to distinguish it from the li.xed 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 Argvie- 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 Harbor an intermittent light which was produced by a beautifully simple contrivance for suddenly lowering and raising a gas-flame. Stevenson projjosed an intermittent light of uneqiwl periods by caiising unequal sectors of a spherical min'or to revolve between the flame and a fi.xed 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 1 shall be useful to the mariner is that of refrae- r tion. known as the dioptric si/steni. If a flame be placed in the focus of a lens of the proper f(U-m. the diverging rays will be bent parallel to each other, so as to form a single solid beam of light. Fresncl 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- I strument by ButVon and Condorcet. He also, in ' conjunction with Arago and ilathieu. used a large lamp having four concentric wicks. In or- der to produce a revolving light on the lenticular or dioptric system, a dilTerent arrangement was adopted from that already described for the catojitric system. The large lamp was now made a fixture, and four or more annular lenses were fitted tiigcther. 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 axi.q 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 Fresnel. These consisted of a pyramid of lenses with mirrors placed above at the proper angle for rendering the rays passing upward jiarallel 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 be generated were the middle vertical profile of an annular lens made to revolve around a vertical axis. 'Hie actiim 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 fi.xed lights by parabolic reflection. It is obvious, however, from our description that the diverging rays which were not interce]>ted by this cylindric hoop, or those which would have passed upward and been uselessly expended in illuminating the clouds, or downward in useless- 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 one- fourth (.249) by employing glass prisms instead of metallic reflectors for lighthouse illumination. There were introduced above and Ixdow the cy- lindric refracting hoop separate glass prisms of triangular section, the first surface of each of which refracted to a certain extent any ray of light which fell upon it, while the second surface was placed at such an angle as to reflect, by total reflections, the ray which had liefore been refracted by the first surface; and the last or Fig. 3. VERTICAL SECTIOX OP rRE8NEL 9T8TKM. outer surface produced another refraction, which made the ravs finally pass "out parallel with those refracted bv the central cylindric hoop. The light falling above the cylindric hoop was thus by refractions and reflections bent down- ward, and that falling below was bent upward, so as to be made horizontal and paralled with that proceeding from the refracting hoop. Fig.