162 



LIGHTHOUSE ILLUMINATION. 



disc of light thrown by the apparatus upstairs, 

 that there is nothing like the amount of aberra- 

 tion that there would have been if it had been 

 one great bull's-eye. Here is one of Fresnel's 

 lamps of the fourth order so constructed (fig. 57): 



Fig. 57. 



observe the fine effect obtained by these differ- 

 ent lenses as you see them revolve before you, 

 and understand that all this upper part is made 

 to form part of the lens, each prism throwing 

 its rays to increase the effect, and, although you 

 may think it is imperfect because if you happen 



