164 LIGHTHOUSE ILLUMINATION. 



ness, and if it does not diverge at all, it makes 

 the light so small, that perhaps only one in 

 a hundred can see it at the same time. The 

 South Foreland lighthouse is, I think, 300 or 400 

 feet above the level of the sea, and therefore it 

 is necessary to have a certain divergence of the 

 beam of light in order that it may shine along 

 the sea to the horizon. I have drawn here two 

 wedges, one has an angle of 15, and shows you 

 the manner in which the light opens out from 

 this reflector seen at the distance of half a mile 

 or more, the other wedge has an angle of 6, 

 which is the beautiful angle of Fresnel. When 

 the angle is less than 6, the mariner is not 

 quite sure that he will see the light he may 

 be beneath or above it; and in practice it is 

 found that we cannot have a larger angle than 

 15, or a less one than 6. In order, therefore, to 

 get more light, we must have more combustion, 

 more cotton, more oil ; l but already there are in 

 that lamp four wicks put in concentric rings, 

 one within the other, and we cannot increase 

 them much more, owing to the divergence 

 which would be caused by an increase in the 

 size of the light the more the divergence, the 

 more the light is diffused and lost. We are, 



