Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/643

 LIGHTHOUSE 621 assisted in carrying out the design in 1843. This combination added, however, but little to the power of the flash, and produced both a periodically flashing and constantly fixed light ; but it must be remembered that the prism for fixed lights was the only kind of reflecting prism then known. The combination of trapezoidal lenses and mirrors was also, for the same reason, still used. The prisms for Sker- ryvore were the first that were made of the large size (first order), and were constructed by M. Soleil at Paris under the superintendence of Leonor Fresnel, the successor of his brother Augustin, then deceased. Besides the designs which have been described for improv ing Fresnel s revolving light, there were many others, among which may be mentioned those of Leonor Fresnel, Sir David Brewster,W. H. Barlow, F.R.S., A. Gordon, Lepaute, Letour- neau. Reynaud, and Tabouret ; but in all these, as well as in ; FlG - 26. Vertical Section, those which have been described, the rays were either not parallel ized in every plane or else unnecessary agents were resorted to for that purpose. T. Stevenson s HOLOPHOTAL System. Holo- It was not till 1849-50 that the problem was first solved of photal condensing the whole sphere of rays diverging from a flame into system, a single beam of parallel rays without any unnecessary reflexions or refractions. Catadioptric Holophote. In figs. 27 and 28 part of the anterior hemisphere of rays is intercepted and at once uarallelized by the FIG. 27. Vertical Section. FIG. 28. Front Elevation. Cata- lens L, whose principal focus (i.e., for parallel rays) is in the centre dioptric of the flame, while the remainder is intercepted and made parallel by the paraboloid a, and thus the double agents in Fresnel s design are dispensed with. The rays of the posterior hemisphere are re flected by the spherical mirror b back again through the focus, whence passing onwards one portion of them falls on the lens and the rest on the paraboloid, so as finally to emerge in union with and parallel to the front rays. This was the first instrument which intercepted and parallelized all the rays proceeding from a focal point by the minimum number of agents. It is therefore geometrically per fect, and was called by Mr Steven son a holophote, but it is not physi cally so, for it employs metallic reflexion, and with an ordinary oil flame and burner some of the rays reflected by the spherical mirror would fall upon the burner and be lost. This instrument was first employed at the North Harbour of Peterheadin 1849. Ilolophotal Catadioptric Appa ratus Revolving round a Central Flame. If in place of Fresnel s compound arrangement of trape zoidal lenses and piano mirrors FIG. 29. Vertical Section. there arc substituted mirrors R, R (fig. 29) generated by the revolu tion of a parabolic profile round a horizontal axis, all the light will ]&amp;gt;e at once sent out in parallel beams by them and the lenses L, and the apparatus is therefore geometrically perfect, but metallic instead of glass agents are still employed. T. Stevenson s Ilolophotal Dioptric Agents. The agents for rendenng the holophote purely dioptric fall next to be described. Section on a t. Holophotal Prisms, 1850. If prisms are generated by the revolu- Holo- tion of sections of Fresnel s prisms round a horizontal instead of a photal vertical axis, as shown in fig. 30, rays incident upon them will prisms. be made parallel in every plane instead of in the vertical plane only as in Fresnel s prisms.* In 1852 it was stated that A. Fresnel, in 1826, tried prisms of this kind on lamp-posts for the quays of a canal in Paris, but their use was discontinued. They were not so arranged as to be applicable to lighthouses, and no account of them was published. Double Reflecting Prisms giving Two Internal Reflexions instead of One, 1850. The object of these prisms is to reflect the rays back through the focus. The rays from/ (fig. 31) pass normally through Fig- 30. FIG. 32, Vertical Section. FIG. 31. Vertical Section. the surface be and fall upon the surfaces ab, ac, by which they are totally reflected and sent back to the flame. 2 Optical Combinations of Dioptric Holophotal Agents. Dioptric Holophote with Dioptric Spherical Mirror. If rings of Dioptric holophotal prisms p, p (fig. 32), combined with a central refracting holophote lens L subtending together 180, be placed before a flame, the whole with of the front half of the diverging sphere of rays will be at once mirror, condensed by refraction and total reflexion into one beam of parallel rays. Next, if there be placed behind the flame a dome of glass a formed of zones generated by the revolution of the cross section of the double reflecting prisms round a horizontal axis, the back hemisphere of rays will, after the double reflexion, be re turned by it through the flame so as to diverge along with tho front rays ; for this dome is a perfect mirror, not only for the faint light that is superficially reflected by the first concave surface, but also for the main portion of the rays which enter the substance of the glass and are there totally reflected. Thus the whole light is paral lelized entirely by glass agents. This com bination should therefore produce the light of maximum intensity, being both geometrically and physically perfect when the radius is a luminous point. Still, some of the rays reflected by the upper and lower parts of the mirror will not clear the burner of an oil light, but falling on it will be lost. Dioptric Holophotal Revolving Light. Application of Total Revolving Reflexion to Revolving Lights, 1850. If sections of the front half of light, the dioptric holophote be arranged as a polygonal cage round a large central flame and made to circulate round that flame, a revolving light with its periodic flashes will be pro duced (fig. 33), which is therefore geometrically and physically perfect, and the double agents used in Fresnel s revolving light are thus wholly dispensed with. Mr J. T. Chance says, &quot;on the whole the modern plan (holophotal) must give light five or six times more intense than that of the former (Fresnel s) arrangement.&quot; The greater sim plicity and compactness of this light may be seen in comparing figs. 21 or 26 with fig. 33. The holophotal revolving light is now the only ono that is employed for all new lights. It was first intro duced in 1850 on the small scale by Messrs Stevenson at Horsburgh Rock, Singapore, and on tho larger scale at North Ronaldshay in Orkney, the prisms of which were most successfully made in 1851 by Letourneau of Paris. FIG. 33. Elevation. 1 Trans. Roy. Scot. Sac. of Arts, 1850. 2 Kid.