184 FKESNEL. 



railway locomotive at night, making the light far more 

 effectual on the track ahead than if a simple lamp without 

 a reflector, however bright it may be, were vised. 



For we must remember that light, in radiating from the 

 luminous point through the atmosphere, loses brilliancy as 

 the distance increases, from two causes; first, from the 

 spreading or diffusion of it, on account of the divergence 

 of the rays from each other as they recede, which causes 

 the intensity of it to diminish, as we saw in a former chap- 

 ter, as the squares of the distances; and, secondly, on ac- 

 count of the interception of the light by solid or liquid par- 

 ticles always floating in the atmosphere, which, though in- 

 dividually invisible to us, absorb in the aggregate a great 

 deal of light, especially when the distance through which 

 the ray has come is great, so that it has had to encounter 

 a great number of them. 



Now there is no means of preventing the loss of light 

 from this latter source, namely, the absorption of it by sub- 

 stances floating in the air and thus diminishing the trans- 

 parency of it. All that can be done is to increase the 

 quantity of light sent forward, so that the distance may be 

 greater that will be required to absorb it all. But the 

 former that is, the diminution by divergence may be in 

 a great measure controlled by means of lenses or reflectors 

 so arranged as to collect the light from all sides, and send 

 it forward over the sea in rays diverging but little later- 

 ally, and lying, horizontally, very nearly in the same plane. 

 It is evident that they must not be in precisely the same 

 plane, for the surface of the sea is not itself plane, but 

 slightly convex, on account of the rotundity of the earth. 

 The rays, of course, can not be made to curve in their 

 course to accommodate themselves to this rotundity, and 

 so it is necessary that they should diverge a little upward 

 and downward in order that they may shine upon both 



