3 represents in vertical section this, which is the most perfect of Fresnel's inventions in light house illumination, especially when made in pieces of the rhomboidal form, and used in con nection with a diagonal framing introduced by Allan Stevenson. In Fig. 3 it shows the re fracting and totally reflecting prisms, r the ey lindrie refractor, and f the radiant. From what has been stated, it will lie readily seen that, in so far as regards fixed lights, which are required to illuminate constantly the whole of the horizon with intensity. the Ilioptrie light of Fresnel with Allan Stevenson's improvements is a perfect instrument. But the case is different as regards revolving lights. or those where the whole rays have to he concentrate() into one or more beams of parallel rays. To revert to the parabolic re flector, it must be obvious that all rays which escape past the lips of the reflector never reach the eye of the mariner, while, if we return to the dioptric revolving light of Fresnel, we find that those rays which escape past the lens are acted on by ItC0 agents, both of which cause loss of light by absorption. The loss occasioned by the inclined mirrors, and in passing through the pyramidal inclined lenses. was estimated by Fresnel himself at onr-hulf of the whole incident rays. In order to avoid this loss of light, Thomas Stevenson proposed in 1849 to introduce an ar rangement by which the use of one of these agents is avoided, and the employment of the total reflection which had been successfully em ployed by Fresnel for fixed lights was intro duced with great advantage for revolving lights. Ile combined an annular lens, L (Fig. 4) ; a parabolic colloid, a. truncated. at its parameter, or between that and its vertex; and a portion of a spherical mirror, b. The lens and mirror were placed at the proper focal distance from the flame. so that all the rays of light own ing from the front of the flame were intercepted either by the paraboloid or the lens. The rays emanating from the flame may he regarded as divided into two hemispneres. The lens intercepts part of the anterior hemisphere of rays. and makes them parallel while the remainder fall on the paraboloidal surface, and are reflected parallel. The rays forming the posterior hemisphere fall ing on the spherical mirror are reflected forward again through the' focus in the same lines. but in opposite directions to those in which they came, and then are in part refracted by the lens, and the rest are made parallel by the paraboloid.
The result is that rays finally cooling from the posterior hemisphere emerge horizontally com bined with those from the anterior hemisphere.
Thus the entire sphere of diverging rays is col lected into one beam of parallel rays without employing, any unnecessary agents.
What has been just described was termed by Stevenson a «rtopiric 11°1°1)110e. What follows is a description of his dioptric halophutc, iu which total reflection is the underlying principle. The front half of the rays is operated upon by totally reflecting glass prisms, similar in section to those applied by Fresnel for fixed lights; but, instead of being curvilinear in the horizontal plane only, they are also curvilinear in the verti cal plane, and thus produce. in union with an annular lens, a beam of parallel rays similar to what is produced by the parabolic mirror. The rays proceeding backward fall upon glass prisms, which produce two reflections upon each ray, and cause it to pass hack through the flame, so as to fall ultimately in the proper direction upon the dioptric holophote in front. so that the whole of the light proceeding from the flame is thus ultimately parallelized by means of the smallest number and the best kind of optical agents. It is a remarkable property of the spherical mirror that no ray passes through it, so that an ob server, standing behind the instrument, perceives DO light, though there is nothing between him and the flame hut a screen of transparent glass. Where the light is produced by great central sta tionary burners, the apparatus assumes the form of a polygonal frame, consisting of sectors of lenses and holophotal prisms. which revolves round the flame, and each face of which produces a bean) of parallel rays. Bence, when the frame revolves round the central flame the mariner is alternately illuminated and left in darkness, ac cording as the axis of each successive face is pointed toward him or from him. In the re volving holophotal light one agent is enabled to do the work of two agents in the revolving light of Fresnel, as total reflection, or that by which least light is lost, is sobstituted for me tallic reflection. The dioptric holophotal system, or that by which total reflection is used as a portion of the revolving apparatus, was first em ployed on a small scale in 1850 at the Ilo•sburg lighthouse, and on a large scale in IS.51 at North Bonaldshay in Orkney. Since that date this system has been all hut universally intro duced into Europe and America.