/fl 2. 



CHAPTER I. 



RADIATION. 



LIGHT proceeding from a luminous object tends to radi- 

 ate in all directions. If the luminous object is a candle, 

 the rays can only diffuse themselves upward and on the 

 sides, those tending downward being intercepted by the 

 candlestick, the table, and the ground. 



If the candle, so shining, is supposed to be at the surface 

 of the earth, or upon any horizontal plane, and there is 

 nothing to intercept its rays upward or on any side, then 

 it is plain that the space which the rays illuminate will be 

 of the form of a hemisphere, with a radius equal to the dis- 

 tance through the air to which the light could penetrate. 

 The base of the hemisphere would coincide with the ground, 

 or the horizontal plane, whatever it might be, on which 

 the candle was placed, while the spherical surface of it 

 would extend into the air, forming a great dome over and 

 around the candle, like a kind of lower sky. 



Let us suppose that the atmosphere at the time in ques- 

 tion is so clear that the light of such a candle would be 

 visible for a distance of half a mile. Then the radius of 

 the hemisphere in the atmosphere which would be illumin- 

 ated that is, the distance from the centre to the outer 

 boundary of it, would be half a mile. 



