Refrangibility of Light. 157 



figure will be represented, its motions, its colours 

 and shadings. 



The whole of the preceding theory may be 

 illustrated by means of a common reading-glass. 

 If a candle is held so near it, as that the rays 

 passing through shall strike the wainscot of the 

 chamber with a bright spot, just as large as the 

 glass itself, the candle is then at the focal dis- 

 tance; and rays, striking the glass divergently, 

 are refracted through it, parallel to each other, 

 neither spreading nor drawing together as they 

 proceed. If the candle is held nearer than the 

 focal distance, the rays will fall then more di- 

 vergent upon the glass, and will consequently be 

 refracted more divergent, so that they will form 

 a very broad spot of light upon the wainscot. 

 If the candle is placed at a much greater distance 

 than the focus, the rays fall upon the glass more 

 nearly parallel, and consequently, when they 

 are refracted will tend to unite and converge 

 behind the glass, and will form but a small speck 

 of vivid light on the wainscot. This speck, if 

 closely examined, will appear a perfect picture of 

 the candle. 



Every visible point, in any body whatever, 

 may be considered as a candle sending forth its 

 rays, which split and pencil out into several other 

 rays before they arrive at the eye. Each body is 

 as if composed of an infinite number of splendid 

 points or candles, each point with its own radi- 

 .ance, .and diffusing itself on every side. Instead 



