Mr StevensorCa Totally 'Reflecting Mirfor. 143 



As there is positively no limit to the extent to which the 

 brightness of sea-lights might he usefully increased, it is 

 evident that, although we were in possession of a practically 

 available source of illumination many times more intense 

 than that which we actually employ, it would still he an 

 object of the greatest importance to use it in the most advan- 

 tageous manner. 



My friend, Mr Thomas Stevenson, in his recently invented 

 holophotal arrangements, has afforded the means of transmit- 

 ting all the rays emanating from a flame in a single direction ; 

 and a beam of light proceeding from one of them should, there- 

 fore, be brighter than that transmitted by any other species of 

 lighthouse apparatus not fulfilling the same condition, pro- 

 vided the intensity of the source of light is the same in both 

 cases. 



This paper has relation, not to the holophotal system of 

 illumination generally, but simply to a new and somewhat 

 singular species of reflector, invented by Mr Stevenson, and 

 which forms part of his apparatus. 



The use of this reflector will be understood from fig. 1, which 

 represents a vertical section of one of Mr Steven son's holopho- 

 tal arrangements. The holophotal apparatus in every variety 

 of form consists essentially of two parts. One, D G H E, 

 placed before the flame, which, in the apparatus shewn in the 

 figure, is a series of totally reflecting prisms, with a central 

 polyzonal lens, possesses the property of transmitting all 

 the rays, F g, falling upon it in parallel directions, g h. 

 The other, D B E, is a hemispherical mirror placed behind, 

 with the flame in its centre ; and its use is to collect all the 

 remaining rays, which otherwise would be lost, and to re- 

 flect them back through the flame, so that they may fall on 

 the apparatus in front in such directions as to admit of their 

 being finally transmitted in the same parallel beam with the 

 other rays. 



Now, if this reflector were composed of polished metal, 

 a large portion of the incident light would be lost by absorp- 

 tion ; and Mr Stevenson conceived the ingenious idea of sub- 

 stituting for a metallic mirror, a series of prisms of glass, by 

 which the light, after suff'ering two reflexions, might be re- 

 turned back to the radiant point. The object of this paper 



