FADING ILLUMINATION 



241 



artificial light necessary, and even during the day artificial 

 light is needed in buildings whose structure excludes the nat- 

 ural 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 oil lamps can 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. 



Fading illumination. 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 rap- 

 idly than we realize, as the 

 following simple experiment will show. Let a single candle 

 (Fig. in) serve as our light, and at a distance of one foot from 

 the candle place a photograph. In this position the photo- 

 graph receives a definite amount of light from the candle and 

 has a certain brightness. 



FIG. in. A photograph at a receives four 

 times as much light as when held at b. 



