CHAP. II.] 



LIGHTHOUSES. 



229 



eliminate metallic reflection from the system, the dioptric-holophote 

 shown in Fig. 163 is employed, in which total reflection takes the 

 place of metallic. L is Fresnel's annular lens,pp are totally reflect- 

 ing prisms, similar in section to those invented by Fresnel for fixed 

 lights, but which are generated by revolution of those sections round 

 a horizontal instead of a vertical axis, and have therefore the property 

 of parallelizing the rays from the flame in every plane, instead of in 

 the vertical plane only, .as in Fresnel's fixed lights. These prisms in 



FIG. 163 Dioptric hoJophote. 



FIG. 164.- Section of dioptric spherical prism. 



connection with the central lens render parallel one-half of the 

 rays. The rays passing backwards fall upon the prisms of double 

 agency a l>, and instead of passing through them, as in all other light- 

 house prisms, they are after two total reflections sent back through 

 the flame, as shown in Fig. 164, in which the dotted line represents 

 the path of the rays and subjected to the parallelizing agency of the 



