50 LABORATORY EXERCISES 



squares each 4 cm. on a side. Darken the room, and use a 

 small " birthday" candle as the source of light. 



Hold the small square upright, 10 cm. from the candle flame, 

 and hold the ruled paper beyond it, and at such a distance that 

 the shadow of the small square just covers 4 of the squares 

 on the ruled paper. Measure the distance of the ruled paper 

 from the flame, and compare it with the distance of the small 

 square from the flame. Give the results. 



b. Hold the cardboard square as before, 10 cm. from the 

 flame, and hold the ruled paper so that the shadow of the small 

 square covers 9 squares on the ruled paper. Compare the 

 distances as before. 



Again change the position of the ruled paper so that all 16 of 

 the squares are covered by the shadow of the small square. 

 What is the distance between the ruled paper and the candle 

 now? 



c. How many squares are there on a checkerboard? If you 

 were to cut a cardboard square having the size of one of the 

 checkerboard squares, and were to hold it upright, 10 cm. from 

 the candle flame, how far from the candle would the checker- 

 board need to be so that the shadow of the single square could 

 cover all the squares of the board? 



EXERCISE 49 

 CANDLE POWER 



Apparatus and Materials. Large and small candles of Exercise 47, 

 two spools of thread, white paper, lamp or electric bulb, metric rule. 



a. In a dark room use again the two candles, the spools of 

 thread, and the white paper of Exercise 47. Place the spools 

 (one upon the other) between the two candles, and note whether 

 the two shadows are equally dark. If not, shift the spools until 

 this is the case. Then measure the distance from the spools to 



