14 RADIATION. 



But, now, if we suppose that instead of a candle at the 

 surface of the earth we have a flame, or other incandescent 

 object, of the same size, and of precisely the same power 

 to radiate light, in the air half a mile above the surface 

 of the ground, then the hemisphere that was illuminated 

 would become a sphere or globe, the diameter of which 

 would be a mile, the distance from the centre to the cir- 

 cumference being on every side half a mile. 



The light of this candle, or of the luminous object, what- 

 ever it might be, so placed, would be barely visible to any 

 one on the earth looking upward, for, by the supposition, 

 half a mile is the limit of the distance to which the rays 

 could penetrate through the atmosphere and retain suffi- 

 cient force to produce their proper eifect on the human 

 eye. An eye placed any where else, also, on the margin of 

 the illuminated sphere, and directed toward the centre, 

 would see the light. So, also, if an eye were placed any 

 where within the outer boundary of the sphere, and were 

 directed toward the centre of it, it would see the light, the 

 impression being the more vivid as the eye making the ob- 

 servation moved in from the outer boundary toward the 

 centre. 



We must not suppose, however, that such an illuminated 

 sphere as we have described would have any precise or 

 definite boundary. Some human eyes are much more sen- 

 sitive than others, and can see a much fainter light, or, in 

 other words, can see a given luminous object at a much 

 greater distance than others. The eyes of some animals, 

 such as insects, night birds, or beasts of prey, are probably 

 more sensitive than any human eyes. And even beyond 

 the limit at which the light would cease to produce an ef- 

 fect upon any organ of vision, some of its radiations may 

 penetrate and produce other effects of which we have no 

 cognizance. So that the magnitude of the sphere which 



