with which flashing gas Hghts, un- 

 attended, may be established in 

 that region, where it would be 

 difficult and expensive to main- 

 tain keepers. At stations, how- 

 ever, where there are fog signals, 

 keepers must be stationed, as there 

 is not yet available a practical 

 automatic fog signal for land use. 



POWERFUL REFLECTORS, LENSES, 

 AND PRISMS ARE USED 



In order to increase the effec- 

 tiveness of illumination, reflectors, 

 lenses, and prisms are used to con- 

 centrate the light and throw it out 

 either in a plane around the hori- 

 zon or in a beam or limited arc, 

 where it will be most useful. 

 Parabolic reflectors were intro- 

 duced about 1763, and to show 

 around the horizon or to render 

 the light more powerful it was 

 necessary to mount on a chan- 

 delier a number of lamps each 

 with its own reflector. Thus in 

 an early list of American lights 

 the number of lamps is given, as 

 Boston lighthouse 14 lamps, and 

 Sandy Hook 18 lamps. 



The French physicist, Augustine 

 Fresnel, beginning in 1822, revo- 

 lutionized lighthouse practice by 

 inventing a system of annular 

 lenses, refractors, and reflecting 

 prisms, all of glass and surround- 

 ing a single central lamp. Various 

 forms of lenses designed on these 

 principles, with further improve- 

 ments, are now universally used 

 in lighthouse work, varying from 

 the simple lens lantern, with a 

 single annular lens, to the great 

 first-order lenses, built of many 

 pieces of beautifully cut and pol- 

 ished glass. 



Of such a lens the distinguished 

 lighthouse engineer, Alan Steven- 

 son, wrote : "Nothing can be more 

 beautiful than an entire apparatus 

 for a fixed light of the first order. 

 It consists of a central belt of re- 

 fractors, forming a hollow cyl- 

 inder 6 feet in diameter and 30 

 inches high; below it are six tri- 

 angular rings of glass, ranged in 

 a cylindrical form, and above a. 



36 



