LIGHTHOUSE 623 the range, or distance at sea from which the light can be seen, and DYWLD the solid horizontal angle that is to be illuminated and into which all the light DAD which would naturally diverge over the rest of the circle must be compressed, and over which it must be uniformly spread, ted Requirement! for FIXED Condensing Lights. 1. Where alight has uli-ns- to be seen constantly over only one arc of the horizon, the apparatus must compress all the rays within that one arc whatever its ampli tude may be, nnd spread them uniformly over it. 2. Where the whole horizon has to be constantly illuminated, but the light has to be seen at greater distances over some parts of the sea than over others, as much light must be abstracted as can be spared from the shorter ranges and diverted to the longer so as to allocate the rays in the compound ratio of the number of degrees and the squares of the distances from which the light has to be seen in each arc ; and the light thus diverted from one arc to strengthen another must be spread uniformly over the one that has to be strengthened. By this mode of abstraction and addition there is produced a constant equitable distribution over the whole horizon of all the rays from any single flame. Where the whole horizon has not to be lighted, and where more than one arc has to be strengthened, the rays which would be lost on the land or can be spared in some other direction must be diverted to and spread uniformly over these arcs in proportion to their amplitudes and ranges. volv- Requirements for REVOLVING Condensing Lights. 1. Where a , con- light has to give its flashes periodically over only one arc of the ising horizon, all the rays must be collected and sent out periodically in lits. solid beams of equal power over that arc. 2. Where the whole horizon has to be periodically illuminated, but where its flashes have to be seen at greater distances over some parts of the sea than over others, the apparatus must be made (as in number 2 of the fixed condensing light) to vary proportionally the power of the flashes whenever they begin to pass over those parts of the sea where the ranges are of different lengths so as to produce an equitable periodic distribution of all the rays. 3. Where the whole horizon has not to be lighted, and where more than one arc has to be strengthened, the rays which would be lost on the land or can be spared in some other direction must be diverted to and spread uniformly over these arcs so as to strengthen the flashes in proportion to their amplitudes and ranges. If n be the number of degrees in an arc to be illuminated, and d the distance in miles to be traversed by the light, then, neglecting atmospheric absorption, the quantity of light to be allocated to that arc will be proportional to nd 2 ; but if we take account of atmo spheric absorption, supposing q to be the quantity out of a unit of transmitted light which escapes absorption after passing through a mile of air, then the whole light needed by the arc to be illuminated will be proportional to m = nd-q d . Supposing now that L is the whole 360" of available light from the apparatus, the quantity to be apportioned to the .cjiven arc will be iL/22, where 2/;i denotes the sum of the severnl numbers m computed for the respective arcs of the horizon. The Pri,u:ipal Optical Condensing Agents. mdens- Condensing Straight Prisms. These, either by reflexion or refrac- ? tion or both, cause a ray fr (fig. 40) proceeding in any compass might STf"! r j i ; Section U!. 1 ^ 1 ! L /57 flf h on ah. ght- gled Fig. 40. bearing from a fixed light apparatus AA to emerge in the direction, e.g., parallel to the corresponding ray fb, which proceeds in the same compass bearing from another part of the apparatus and so of any other ray/c which is bent parallel to the ray fa. Right-Angled Expanding Prisms. These (fig. 41) are right-angled vertically, while in piano two of them CjC., are semi-rings, and the third C 3 a semi-cone. A semi- cylindric beam of parallel rays passing vertically upwards normally on the bases a of the prisms enter the glass, a and falling are reflected Twin prisms. FIG. 42. Section. FIG. 45. Horizontal Section. by the sides b, and pass out horizontally and normally to the other sides ; but, as the prisms are bent through a circular segment d ad in piano, the emergent rays will be spread over the same angle in azimuth, and this will be true of any angle in azimuth subtended by the prisms. As those in the diagram subtend 180, the light will in this case be spread over half M&f. the horizon d ad. Twin Prisms. These are for carrying out Pro fessor Swan s mode of causing light coming from prisms behind to pass through spaces left for the purpose between others in front. The twin prisms (fig. 42) are formed by cutting out the apex (shown black on diagram) of a straight prism so as to leave a sufficient space between it and the next prism for the passage of rays coming from behind. The length of glass traversed by the rays is lessened by this arrangement, and the size of the apparatus and lantern can at the same time be very greatly reduced. Differential Lens. Horizontal divergence may be obtained to any Differen- required amount by varying the radius of curvature of the inner tial leiis face of an annular and re- lens. The outer face ^^^ ^^v c ^ faw c fractor. (fig. 43) is the same as that of an ordinary annular lens, while the other face (fig. 44), though straight in the vertical, is ground to the re quired curve in the horizontal plane. The rays /7c (fig. 45) fall ing upon the lens x converge to the verti cal focal plane ff and afterwards diverge through the smaller horizontal angleo//V, and so for any other case. Differential Refrac tor. This is the ap plication to the cylin- dric refractor of the same principle which has been described above for the ordinary annular lens. Condensing Catoptric Spherical Mirror. If a large arc of a Spherical spherical mirror be cut opposite to a short range of sea, and a mirror, smaller arc be cut in the mirror opposite to the longer range which has to be strengthened, then the light passing through the larger cut is received on an elliptic reflector placed behind, so as, with the help of an additional reflector and lens, to cause the larger cone of rays to be compressed into the smaller arc which has to be strengthened. Spherical Mirror of Unequal Area. This mirror is cut down in height in such a way that its different heights represent inversely the different distances of the neighbouring land from the lighthouse, so as to reflect less light in the shorter and more in the longer ranges. Application to Fixed Condensing Apparatus. Fixed Condensing Light for a Single Sector, 1850. Theholophote Light for pLp (fig. 46) throws its whole light on straight condensing prisms single c, each of which distributes the _ sector, rays over the required sector. Condensing Apparatus for Steamers Side Lights. Bymeans of this application of the con densing principle (figs. 47 and 48) all the light can be dis tributed with strict equality over 112 30 , which is the arc prescribed for steamers by the Board of Trade. Several of the Transatlantic and other steamers have adopted this kind of ap paratus, which is hung on gim bals and placed in iron towers, having an entry from below the deck, which can be made use of in bad weather. Condensing Quadrant. The fixed apparatus bbb, with spherical Condens- mirror behind, throws its rays directly through the angle of 90 ing quad- afa , while the supplementary rays falling on the straight condens- rant, ing prisms p, p, p , p (fig. 49) are sent out parallel to the corre sponding rays in the unobstructed central quadrant of the main apparatus. The whole light will therefore be condensed equally over 90. Steamers side lights. FIG. 46. Vertical Section.
