622 LIGHTHOUSE Fixed light with flashes. Back prisms. Holophotal Fixed Light varied by Flashes. Fresnel s double agents (figs. 22, 23) are here also dispensed with by the single agency of panels of fixed light apparatus p , p (fig. 34) and cylindrical refractors L , L , alternating with panels of holophotal apparatus p, p, L, L, both of which revolve together round the central burner. Hack Prisms. It will be noticed that, while perfection was attained by single agency in Fresnel s fixed apparatus and also in the holophotal revolving apparatus, a physical de fect still attached to every form of holophote which compressed the light into a single beam where the rays require to be sent back through the flame. The defect is not a geometric one, and would be non-existent were the luminant a mathematical point. In order to remedy this objection, Mr T. Stevenson and Mr Brebner designed what they termed "back" prisms capable of deviating the rays through an angle of 130, so that the engineer becomes virtually in dependent of the critical angle. FIG. 34. Vertical Section. Professor Swan of St Andrews also independently proposed the same kind of prism, accompanied by general formulae for its construction. The ray ab (fig. 35) is refracted at b, totally re- Circular form.. Section. "# 8^ Slraiffht form.. Elevation Sections U Fig. 35. fleeted at c, and again refracted at d, so as to pass out parallel to the horizontal axis. These prisms may be formed by the revolution of the generating section round either a vertical or horizontal axis, or they may be made straight as shown in elevation and section in the diagram. The " back prisms" which were first used in Islay, Argyleshire, were made by Messrs Chance in accordance with Professor Swan s formula. Perfect Perfect, Form of Dioptric Holophote form of for an Oil Flame. By combining holo- the back prisms ga, he (fig. 36) just J phote. described with a semi-holophote abc subtending 180 and a portion of the dioptric spherical mirror ijk, no light is lost on the burner, and all the rays are parallelized, so that this apparatus, being all of glass, is both geometrically and physically perfect. Chance s J. T. Chance s Improvement 0/1862 Fia 36. Vertical Section, improve- on Stevenson s Dioptric SpJicrical Mirror. Mr Chance says, " The ment. plan of generating zones round the vertical axis (instead of the horizontal) was introduced by the author, who adopted it in the first Seclicrv cn/Une> AB Fig. 37. complete catadioptric mirror which was made and was shown in the exhibition of 1862 by the Commissioners of Northern Lighthouses, for whom it was constructed, in order to further the realizing of what Mr T. Stevenson had ingeniously suggested about twelve vears pre viously. During the progress of this instrument the idea occurred to the author of separating the zones (fig. 37) and also dividing them into segments like the ordinary reflecting zones of a dioptric light ; by this means it became practicable to increase considerably the radius of the mirror and thereby to render it applicable to the largest sea light without overstepping the limits of the angular breadths of the zones, and yet without being compelled to resort to glass of high refractive power." Mr Stevenson states (Min. Civ. Eng., vol. xxvi.) that he origin ally intended to generate the prisms round a vertical axis, but adopted the horizontal as more practicable where flint glass which was taken out of the melting pot on the end of a rod had to be employed for the most important and smaller central parts of the apparatus, which at that date were the only ones that could be constructed. Professor Swan s Designs. Among several ingenious arrange ments and new forms of agents proposed by Professor Swan 1 is the mode of sending rays from prisms through inter stices left between other prisms placed in front, and also a form of agent which he termed the triesoptrie prism, in which the rays would undergo two refrac tions and three reflexions. In fig. 38 a are the front and b the triesoptrie prisms. The two upper and lower prisms a are constructed of flint glass of high refractive power. It will be observed from the drawing that this ingenious arrangement is nevertheless open to objec tion, for cones of light of 30 in front and of 65 at the back are lost through the interstices. Swan s designs. FIG. 38. Vertical Section. T. Stevenson s Azimuthal CONDENSING SYSTEM for Distributing the Light Unequally in Different Directions cither Constantly or Periodically. Previous to 1850 all apparatus was designed to emit light of Conclens- equal power in every azimuth either constantly or periodically, so ing that when applied to illuminate seas extending farther in some system. directions than in others, such, for example, as long narrow sounds, the power was either needlessly great in some azimuths or not nearly strong enough in others. The only exception was where a tower was placed on a long straight coast, when a spherical mirror was fixed behind the flame to prevent the light passing landward. The light that would have been lost on the land was, in this way, sent seawards, but no attempt was ever made to condense the light over a given arc or to allocate this auxiliary light in proportion to the varying lengths of the different ranges and the amplitudes of the arcs over which it had to be seen, nor, where a light had to show all round the horizon, to weaken its intensity in one arc and with the rays so abstracted to strengthen some other arc which from its range being longer ought to be of greater power. As the optical agents which we have described were insufficient for this purpose, it became neces-. sary to devise new agents pos- sessing special optical properties^ for distributing the rays not equally, but equitably. Some of the more important of these agents and their combinations will be described, but before doing so it is necessary to make some preliminary explanations. The form of the emergent light from condensing ap paratus is neither a solid beam of parallel rays like that from an annular lens nor yet a zone of rays diverging naturally in azimuth all round like that from an ordinary fixed apparatus, but is intermediate between these, being a solid angle or wedge of light strengthened by those rays which would naturally diverge in other directions, but which are diverted and spread over the given sector as shown perspectively in plan in fig. 39, in which L represents the position of the lighthouse, LD Fig. 39. 1 " Professor Swan s Designs," Trans. Roy. Scot. Soc. of Arts, 1SC7-C8.
