for the purposes of Illumination in Lighthouses. 57 



a similar cone L a, b, by a mirror a b, or by obtaining a conver- 

 ging cone of much greater size, by means of a contrivance which 

 will afterwards be described. 



IV. On the Combination of Lenses with Plain and Spherical Mir- 

 rors, for Fixed and Revolving- Lights. 



From the comparison which has now been made of lenses 

 and parabolic reflectors, it appears that, when the lens is used 

 singly, a very large proportion of the light of the flame is not 

 rendered available. In revolving lights, where two or more lenses 

 are combined, this light may be very advantageously employed ; 

 but in fixed lights, or in lights where only one lens is to be used, 

 it requires to be combined with smaller lenses, and with plain and 

 spherical mirrors, in order to enable us to throw into the paral- 

 lel beam all or most of the rays which flow from the lamp. 



The contrivance which occurred to me for this purpose, and 

 which I published in 1812, has been recently adopted in the new 

 system of illumination introduced into the French lighthouses. 

 It is represented in section, in Plate V. Fig. 1., where F is 

 the lamp or source of light, whose rays it is required to throw 

 into one parallel beam. More than one-half of the sphere of 

 light which radiates from this lamp, viz. GCABDE, is intercept- 

 ed by lenses AB, AC, CG, BD, DE. The cone of rays inci- 

 dent upon the lens AB, which is larger, and has a greater focal 

 length than the rest, fall diverging upon the large lens LL, and 

 are refracted into a parallel beam of light LRLRj*. This beam 

 of light is rendered more intense by the cone FMN, which, fall- 

 ing on the concave mirror GMNF, whose centre is F, is made 

 to converge again to F, from which, diverging a second time, it 



* By the interposition of the second lens AB, a much larger cone of rays is 

 thrown into the main beam by the lens LL than could have been done without AB. 

 VOL. XI. PART I. H 



