for the purposes of Illumination in Light-houses. 59 



first zone of lenses will be close to the circumference of the lens 

 AB, Plate V. Fig. 1 . ; and the second zone of lenses will be 

 close to the first zone, without any space whatever between them. 



The preceding apparatus is intended to be a substitute for a 

 single parabolic reflector ; but when the light is to be seen in se- 

 veral directions, or is required to revolve, then two or more pa- 

 rabolic reflectors are united, back to back. Each of the reflec- 

 tors thus united has necessarily a separate lamp ; but if two or 

 more lenses are used, the same lamp will serve for them all, 

 an advantage of no slight consideration. 



The method of uniting two or more lenses will be understood 

 from Plate IV. Fig. 2., which, if the number of large lenses is 

 only two, will be a horizontal section of the apparatus ; but if 

 the large lenses are four, six, or eight in number, it will be a ver- 

 tical section of the apparatus, room being left at D for admitting 

 the lamp, and at C for the chimney. The parallel beam of light 

 formed by the small lens AB, and the large one LL, is widened 

 by means of the lenses AC, BD, and the mirrors a b, ef, while the 

 opposite parallel beam, formed by the small lens GF, and the 

 large one LL, is widened by means of the lenses CG, DF, and 

 the mirrors c d, g h. In this manner, by increasing the number 

 of large lenses, we may, by means of one powerful lamp at F, 

 throw any number of parallel columns of light into a horizontal 

 plane, and increase the width of these beams, by employing small 

 lenses and mirrors to reflect horizontally the light that would 

 otherwise be cast into the sea, or thrown up into the atmosphere. 



IV. On the Construction of Distinguishing Lights. 



" The methods resorted to for distinguishing one light from 

 another, on the coast, in cases where the distance and bearings 

 by the compass may be so trifling as to render some method of 

 distinguishing them necessary, till of late, was only effected by 



H2 



