106 LIGHT 



distance of one foot from the candle place a photograph. 

 In this position the photograph receives a definite amount of 

 light from the candle and has a certain brightness. 



If now we place a similar photograph directly behind the 

 first photograph and at a distance of two feet from the candle, 

 the second photograph receives no light because the first one 

 cuts off all the light. If, however, the first photograph is re- 

 moved, the light which fell on it passes outward and spreads 

 itself over a larger area, until at the distance of the second 

 photograph the light spreads itself over four times as large 

 an area as formerly. At this distance, then, the illumination 

 on the second photograph is only one fourth as strong as it 

 was on a similar photograph held at a distance of one foot 

 from the candle. 



The photograph or object placed at a distance of one foot 

 from a light is well illuminated; if it is placed at a distance 

 of two feet, the illumination is only one fourth as strong, and if 

 the object is placed three feet away, the illumination is only one 

 ninth as strong. This fact should make us have thought and 

 care in the use of our eyes. We think we are sixteen times as 

 well off with our incandescent lights as our ancestors were with 

 simple candles, but we must reflect that our ancestors kept the 

 candle near them, " at their elbow," so to speak, while we sit at 

 some distance from the light and unconcernedly read and sew. 



As an object recedes from a light the illumination which it 

 receives diminishes rapidly, for the strength of the illumina- 

 tion is inversely proportional to the square of distance of the 

 object from the light. Our ancestors with a candle at a dis- 

 tance of one foot from a book were as well off as we are with 

 an incandescent light four feet away. 



101. Money Value of Light. Light is bought and sold al- 

 most as readily as are the products of farm and dairy ; many 

 factories, churches, and apartments pay a definite sum for 



