136 RAMBLES OF A NATURALIST. 



The following numbers will at once show the 

 superiority possessed by the new mode of lighting 

 when compared with that yielded by Borda's mirrors. 

 A polyzonal lens having a diameter of thirty inches, 

 and illumined by a single lamp with four wicks, 

 transmits the rays to a distance of thirty-six miles ; 

 it projects eight times more light towards the horizon 

 than the best reflector, and the effect which it pro- 

 duces in the direction of its axis is equal to that 

 which would be emitted from 4000 combined jets of 

 gas. 



The lenses which we have described are alone 

 applicable to lighthouses having revolving lights, 

 and in this respect they resemble parabolic mirrors ; 

 but a great advantage of the new system is that it 

 may be equally well used for fixed lights. To 

 adapt it to the latter mode of illumination, all that 

 has to be done is to metamorphose the lens into a 

 ring dilated in the middle, and to arrange above and 

 below it a sufficient number of prisms analogous to 

 those of which we have already spoken. In this 

 manner the light is thrown simultaneously to all 

 points of the horizon, only, instead of being united 

 into a cylinder, it forms a kind of horizontal sheet. 

 It will therefore be readily understood that light- 

 houses with fixed lights cannot present the same 

 area of illumination as those which are provided with 

 revolving lights. In both cases the lamp is sur- 

 rounded by a ring of glass of the same height, which 

 receives nearly the same quantity of light in either 

 case. But while in a revolving light the entire 



