120 VEKY BRIGHT LIGHTS. 



surface, an amount less than one half, though still a very 

 large portion, is thus intercepted. 



Then, moreover, as the light of a light-house is not in- 

 tended to guide travelers by land, all that would naturally 

 shine on the landward side, if it were allowed to have its 

 own way, would be entirely lost. In the same manner, as 

 it is not intended to shine for the benefit of the birds in 

 the air, all that would go upward would be lost. In a word, 

 it is only that comparatively small portion of the sphere of 

 radiance that extends forward over the surface of the sea, 

 and at a small distance above it as high as the deck of 

 any vessel that is of any use for the purpose designed. 



Now in ancient times, when these lights consisted sim- 

 ply of blazing fires on the summit of a tower, all except 

 this small portion was lost; but in modern times means 

 have been found to avoid this loss by bending that portion 

 of the rays that would naturally take a wrong direction 

 into the right one that is, by intercepting all or nearly 

 all those rays which would go back over the land, or down 

 into the ground, or up into the air, and turning them in the 

 direction where their services are required that is, out 

 over the water. This is done by certain extremely inge- 

 nious contrivances, through the effect of which the rays 

 which issue from the source of light are collected on all 

 sides and made to shoot forward over the sea, so that, in- 

 stead of forming a sphere, the range of illumination takes 

 the form of a flat wheel, or, rather, half a wheel, extending 

 forward over the water, and lying very low. 



And inasmuch as we can only see any object when the 

 rays from it enter the eye, we can only see the light from 

 a light-house when we are placed within this range. Thus 

 people on the land behind a light-house would not see the 

 light of it at all, nor would birds in the air. A bird that 

 had alighted on the mast-head of a ship coming in a dark 



