FADING ILLUMINATION 



105 



artificial light is needed in buildings whose structure excludes 

 the natural light of the sun. Artificial light is furnished by 

 electricity, by gas, by oil in lamps, and in numerous other 

 ways. Until modern times candles were the main source of 

 light, and indeed to-day the intensity, or power, of any light 

 is measured in candle power units, just as length is measured 

 in yards ; for example, an average gas jet gives a 10 candle 

 power light, or is ten times as bright as a candle ; an ordinary 

 incandescent electric light gives a 16 candle power light, or 

 furnishes sixteen times as much light as a candle. Very strong 

 large oil lamps can at times yield a light of 60 candle power, 

 while the large arc lamps which flash out on the street 

 corners are said to furnish 1200 times as much light as a 

 single candle. Naturally all candles do not give the same 

 amount of light, nor are all candles alike in size. The 

 candles which decorate our tea tables are of wax, while 

 those which serve for general use are of paraffin and tallow. 

 The standard candle with which all lights are compared is 

 made of spermaceti and has 

 a weight of \ of a pound, 

 and a diameter of J of an 

 inch. 



100. Fading Illumina- 

 tion. The farther we move 

 from a light, the less 

 strong, or intense, is the 

 illumination which reaches 

 us ; the light of the street 



lamp on the corner fades and becomes dim before the middle 

 of the block is reached, so that we look eagerly for the next 

 lamp. The light diminishes in brightness much more rapidly 

 than we realize, as the following simple experiment will show. 

 Let a single candle (Fig. 57) serve as our light, and at a 



FIG. 57. A photograph at a receives four 

 times as much light as when held at b. 



