'' AIIMIIIIIIM ''' whole incident rays. In order to avoid this loss of 0, .4 • light, 3Ir. Thomas Stevenson proposed, in 1849, to.
..., 4 aill..41141P introduce an arrangenient by which the use of one of ' ' these agents is avoided, and the employment of total.
Fig. 1. reflection, which had been successfully employed by Fresnel for fixed lights, was introduced with great advantage for revolving lights.
"This effect may be produced in the case of metallic reflectors by the combination, of an annular lens, L (fig. 2); a parabolic conoid, a, truncated at its parameter, or between that and its vertex; and a portion of a, spherical mirror, b. The lens, when at its proper focal distance from the flame, subtends the same angle from it as the outer lips of the paraboloid, so that no ray of light coming from the front of the flame or can escape being intercepted either by the _.,...., paraboloid or the lens. The spherical reflec- b , ,i „ . 1,17= le 1.9 tor occupies the place of the parabolic con a oid which has been cut off behind the . parameter. The flame is at onee in the cen- Opp/pi...1 ter of the spherical inirror, and in the com- b'''''=----- - mon focus of the lens and paraboloid. The whole sphere of rays emanatino. from the , ,,,,- onommimi W ----- \*:.'.mimlamm.
,. flame may be regarded as divided into two "'"' /lint. hemispheres. Part of the anterior liemi- l 11 a , sphere of rays is intercepted by the lens, and -.........m.
made parallel by its action, while the remain- "wall der is intercepted by the paraboloidal surface, “ and made parallel by its action. The rays Fig. 2.
forming the posterior hemisphere fall on the spherical mirror behind the flame, and are reflected forwards again through the focus in the same lines, but in opposite directions to those in which they came, whence paf..s ing onwards they are in part refracted by the lens, and the rest are made parallel by the paraboloid. The back rays thus finally emerge horizontally in union with the light from the anterior hemisphere. This instrument, therefore, fulfills the necessary . conditions, by collecting the entire sphere of diverging rays into one beam of parallel rays without employing aiw unnecessary agents. ' What has been just described is what 3Ir. Stevenson terms a eatoptrie holophote.
What follows is a description of his dioptrie holophote, in which total reflection, or the most perfect system of illumination, is adopted. The front half of the rays is operated upon by totally reflecting glass prisms, shnilar 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 vertical plane, and thus produce, in union with an annular lens, a beam of parallel rays shnilar to what is effected by the parabolic mirror. The rays proceeding backwards fall titian glass prisms, which produce two total reflections upon each ray, and cause it to pass back through the flame, so as ultimately to fall in the proper direction upon the dioptric holophote in front, so that the whole of the light proceeding from the tlame is thus ultimately parallelized by MCDDS of the smallest num ber and the best kinds of optical agents. It is a remarkable property of the spherical mirror that no ray passes through it, so that an observer, standing behind the instru ment, perceives no light, though there is.nothing between him and the flame but a screen ,,if transparent glass.
1Vhere the light is produced by a great central stationary burner, 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 beam of parallel rays. IIence, when the frame revolves round the central flame, the mariner is alternately illuminated and left in darkness, according as the axis of each successive face is pointed toward Min or from him. In the revolving holophotol light one agent is enabled to do the work of two agents in the revolving light of Fresnel, as total reflec tion, or that hy which least light is lost, is substituted for metallic reflection. The dioptric holophotal system, or that by which total reflection is used as a portion of the revolving apparatus, was first employed OD a small scale in 1830 at the IIorsburg light house, and on the large scale in 1851 at North Ronaldsliay in Orkney. Since that date this system has been all but universally introduced into Europe and America.