242 LIGHT 



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 removed, 

 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 for- 

 merly. 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 take 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 or 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 the 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. 



How light travels. We never expect to see around a cor- 

 ner, and if we wish to see through pinholes in three separate 

 pieces of cardboard, we place the cardboards so that the three 

 holes are in a straight line. When sunlight enters a dark 

 room through a small opening, the dust particles dancing in the 

 sun show a straight ray. If a hole is made in a card, and the 

 card is held in front of a light, the card casts a shadow, in 



