350 



THE SUN'S GIFT OF LIGHT 



and 4 candles, placed close together on a piece of cardboard so that 

 they can be readily moved, about 90 cm. away on the other side. 

 Move these candles back and forth till a position is found where 

 both pieces of wax are illuminated alike. Measure the distance 

 of the four candles from the wax. How many times as far away 

 are they as the one candle ? 



The brightness of the sun's light is so great that even an 

 arc light placed in direct sunlight appears as a dark spot. 

 So great, however, is the sun's distance that the earth re- 

 ceives only a minute portion, less than one two-billionth, of 

 the light and heat it gives out. 



The standard measure for intensities of light is the candle 

 power. This is the light given out by a standard candle, 



FIGURE 99 



which is practically our ordinary No. 12 paraffin candle. 

 The ordinary incandescent electric light is sixteen candle 

 power. No comprehensible figures will express the intensity 

 of the sun, using the candle power as a measure. 



The intensity of light, like that of heat and electricity, 

 and all forms of energy which spread out uniformly from their 

 point of origin, varies inversely as the square of the distance 

 from the source. This rapid decrease in the brightness of 

 light as the distance increases is the reason why so small a 

 change in the distance of a lamp makes so great a differ- 

 ence in the ease with which we can read a book. If we 

 make the distance to the lamp half as great, we increase 



