OPTICAL APPARATUS OF LIGHTHOUSES. 477 May 7, 1867. CHARLES HUTTON GREGORY, Vice-President, in the Chair. The following Candidates were balloted for and duly elected :- HARRYFOOTNER, and WILLIAMAUBONE POTTER, as Members; HENRYSLINGSBY BETHELL, ARMAND BOUQTJII$ GEORGE BROWN MURDOCH,JOSEPH SMITH, and HENRYWAUGH, as Associates. No. 1,180.-“ On Optical Apparatus used in Lighthouses.”’ By JAMEST. CHANCE, M.A., Assoc. Inst. C.E.2 THEfollowing notes are designed to convey a general idea of the chief contrivances which constitute the existing system of Light- house Illumination, and to trace the steps of their development. The subject is of great practical importance, and furnishes an in- teresting application of optical science. A complete sketch of Lighthouse Apparatus would far exceed the due limits of this Paper; and, moreover, the various questions con- nected with it have been systematically treated by the late Mr. Alan Stevenson and by Mr. Thomas Stevenson, M. Inst. C.E., to whose works the Author is indebted, as likewise to the following French sources, namely : ‘ The Micmoire of Augustin Fresnel,’ which was read at the Academy of Sciences in July, 1822 ; ‘ The Report of the French Lighthouse Commission,’ dated September, 1825 ; and the recent ‘ Mdmoire uf M. LQonce Reynaud,’ Director of the French Lighthouse Service. The object of Lighthouse Optical Apparatus is to condense, within a small equatorial zone, the available part of the rays which diverge in all directions from a given source of light ; so that as much of it as possible shall be rendered serviceable to the mariner, in the most effec- tive manner, compatible with the special conditions of each locality. The ordinary source of illumination is the flame of an oil lamp on the Argand principle. A single cylindrical wick is employed in the small harbour lights of the dioptric construction,. and also in nearly all kinds of apparatus which consists of metallic parabolic reflectors. But in dioptric sea lights the burner comprises two or more concentric wicks, four being used in the lamp which belongs to an apparatus of the first order ; and as this arrangement neces- sitates a considerable superabundance of oil beyond what is wanted to 1 The discussion ’upon this Paper occupied portions of three evenings, but an abstract of the whole is given consecutively. 2 The Author was elected Assor. Inst. C.E., May 21, 1867. Downloaded by [ York University] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 478 OPTIUALAPPARATUS OF LIGHTHOUSES. feed the 5me, various methods have been adopted for producing the requisite uniform supply. The oil generally employed is t'he colza ; in the lamp, however, with a single wick, it is being superseded by petroleum, which is cheaper and gives a more intense light than colza. The magneto-electric spark has been successfully applied to lighthouse purposes, and it bids fair to be ultimately adopted at most of the important lighthouse stations which are ready of access and otherwise suitable. This brilliant source of lighthas been continuously used at Dungeness by theTrinity Board since the autumn of 1862, the magneto-electric machine being that of Mr. Holmes. The following remarks, however, upon lighthouse apparatus will refer mainly to the oil lamp as the source of light. THE DIOPTRICSYSTEK The Optical Apparatus which is now being universally adopted for sea lights is of the dioptric kind, first successfully introduced by the eminent Augustin Fresnel. It consists of a structure of glass zones, or segments, which in a complete apparatus envelopes the sphere of light radiating from the central flame, except that portion which is intercepted by the burner or is occupied by its chimney. Fig. l,' Plate 15, which is in a plane of the vertical axis of the system, represents the sections which generate the successive zones, and which are such that all raysdiverging from the principal focus are made to emerge in a horizontal direction. The vertical axis of the burner coincides, of course, with that of the apparatus. In reality the upper, middle, and lower portions of the system have generally different foci. An angle of about 57", which the focal horizontal plane bisects, is acted upon by refraction alone ; but the rays which pass above and below this angle are deflected by internal total reflection. Thegenerating sections may evidently describe zones, either round the vertical axis, or round a horizontal one through the focus. If the vertical axis be that of generation, then all rays from the focus will be parallelized only in meridian planes, and the natural divergence in azimuth will remain, so that an uniform light will be distributed to every point of the compass. This constitutes what is termed a Fixed Light. If, however, the axis of revolution be a horizontal one, the action of the apparatusbecomes lenticular, so that all focal rays will emerge parallelto the axis of generation, which will also be that of the compound lens. All the sections may describe complete rings round the horizontal axis, and this is done occasionally in small appmatus ; but the usual met'hod is to divide the sphere into segments, by meri- * The instrument represented in this figure is of the largest kind, and has a diameter of about G feet, and a height of about 9 feet.-J. T. C. Downloaded by [ York University] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. OPTICAL APPARATUS OF LIGHTHOUSES. 479 dian planes at equal angular intervals, the number of these divisions depending on the particular conditions to be satisfied, as to the recur- rence and duration of the flashes. The interval adopted for most lights is that of 45O, as shown in Fig. 2. But whatever may be this angular division, each segment will send forth its own beam, in which all thefocal rays will be parallel to the horizontal axis of revolution ; and in order to render the series of separate beams serviceableto the mariner, the whole apparatus is made to revolve, so as to exhibit the appearance of an alternating succession of brightness and dark- ness, and hence is derived the designation, Revolving Light. The flame has magnitude ; and it is evident that on every point of the apparatus there is incident a conical beam of light, whose apex is that point, and whose directrix is the corresponding contour of the flame : and, if the ray passing through the focus be termed the axis of each indidual cone, the axes of all the emerging beams will be parallelized, but the conical divergence will remain, though slightly modified, after transmission. This divergence can be di- minished either by increasing the diameter of the apparatus, or by diminishing the size, and therefore the power, of the flame ; but some divergence always remains, and is indeed indispensable both in azi- muth and altitude for revolving lights, and in altitude for fixed ones. The difficulty inthe former case consists in obtaining an adequate horizontal divergence withoutwasting light by useless vertical dispersion. As the emerging light is always divergent, its intensity therefore, in any given direction, is subject to diminntion in the ratio of the square of the distance. The Revolving Light is evidently susceptible of much greater intensity than the fixed one, inasmuch as all the light abstracted in the revolving apparatus from the dark intervals contributes a pro- portionate increase of brilliancy. Thus in a first-order revolving dioptric light of eight sides, as the whole 360” are compressed into eight beams whose divergence is about 59, the mean intensity of the flash will be about eight times that of the fixed first-order light. The intensity, indeed, of the brightest part of the flash in a. hori- zontal plane, as measured by observation, is at least twelve times’ that of the fixed light. In consequence, however, of the necessity of distinguishing lights, the fixed one, although so inferior in power, cannot be dispensed with. The diameter of the largest, or first- order apparatus, is rather more than 6 feet, and that of its qua- druple flame is about 34 inches ; the height of the flame above its blue portion being about equal to its diameter. As the flame is diminished in power and size, according to the requirements of the locality to be lighted, so does it subtend a less angle, and 1 It is assumed that the axes of generation of the upper, middle, and lower divisions of the apparatus coincide, or are in the same vertical plane.--J. T. C. Downloaded by [ York University] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 480 OPTICALAPPARATUS OF LIGHTHOUSES, therefore the opticalapparatus can be proportionately reduced. There are, accordingly, a second-order light, which has a ff ame with three wicks, and a thkd-order one, having a flame with two wicks. Then follows the gradation of harbour lights of different sizes, according to the power required. Now although the flame of a sea light is large, the most effective part of it is comprised within a small compass, and subtends only a small angle at the centre of the lens. Again, the angle subtended in a meridian plane by the greater portion of the sea between the .visible horizon and the lighthouse is also extremely small, so that practically whatever part of the flame sends light to the sea horizon is at the same time illuminating the chief range of the sea land- wards. Thus, suppose the flame to be placed 300 feet above the sea, the distance of the horizon is twenty nautical miles ; and yet fifteen miles from the horizon towards the lighthouse subtend only seventeen minutes, which angle corresponds to about ith of an inch at the axis of the flame in a first-order light.