1851 Great Exhibition: Official Catalogue: Class VII.: Thomas Stevenson
100. STEVENSON, THOMAS, F.R.S.E., 84 George Street, Edinburgh — Inventor.
Reversing light, by which one-half the number of reflectors, and one-half the quantity of oil, are said to be saved. Intended for illuminating any arc of not more than 180 degrees. The intervals of time of illumination are equal within the whole of the illuminated arc, instead of unequal as in the reciprocating light. The reflectors are also of a new form, consisting of parabolic strips of different focal distances.
Ordinary parabolic reflector, rendered holophotal (where the entire light is parallelized) by a portion of a catadioptric annular lens. The back part of the parabolic conoid is cut off, and a portion of a spherical mirror substituted, so as to send the rays again through the flame. All the light intercepted by the annular lens is lost in the ordinary reflector.
Holophotal catadioptric annular lens apparatus (unfinished). This is a combination of a hemispherical mirror and a lens with totally-reflecting zones; the peculiarity of this arrangement is, that the catadioptric zones, instead of transmitting the light in parallel horizontal plates, as in Fresnel's apparatus, produce, as it were, an extension of the lenticular or quaquaversal action of the central lens, by assembling the light around its axis in the form of concentric hollow cylinders.
(The above instruments belong to the Board of Northern Lights.)
[The early method of illuminating lighthouses was by coal or wood fires contained in "chauffers." The Isle of May light was of this kind until 1816. The first decided improvement was made by Argand, in 1784, who invented a lamp with a circular wick, the flame being supplied by an external and internal current of air. To make these lamps more effective for lighthouse illumination, and to prevent the rays of light escaping on all sides, a reflector was afterwards added: this threw the light forward in parallel rays towards such points of the horizon as would be useful to the mariner. Good reflectors increase the luminous effect of a lamp about 400 times: this is the "catoptric" system of lighting. When reflectors are used there is a certain quantity of light lost, and the "dioptric" or refracting system, invented by the late M. Augustin Fresnel, is designed to obviate this defect to some extent: the "catadioptric" system is a still further improvement, and acts both by refraction and reflection. Lights of the first order have an interior radius or focal distance of 36.22 inches, and are lighted by a lamp of four concentric wicks, consuming 570 gallons of oil per annum.— S. C.]