72 



PRACTICAL LESSONS IN SCIENCE. 



that applies to the intensity of heat and gravity, and in fact to 

 all forms of radiant energy. This law may be illustrated by the 

 following experiment: Take a piece of stiff cardboard, or tin, 

 about four inches square, and place it at a distance of four feet 

 from a wall or screen. Now, if a candle be placed in front of this 

 card, a shadow will be cast upon the wall which will be larger or 

 smaller as the candle is moved nearer or farther from the card. 

 If the candle is one foot in front of the card the shadow on the 

 wall will be about sixteen inches square, that is the light which is 

 intercepted by the card if allowed to pass on, would be spread out 

 over a surface sixteen times as large upon the wall, but if the 

 same amount of light is spread over sixteen times as much sur- 

 face in one case as in the other it can be only one-sixteenth as in- 



A 



FIG. 7'. 



tense. At four times the distance from the luminous body in this 

 case the intensity of light is one-sixteenth as great; at three times 

 the distance the light would in the same way be found to be one- 

 ninth as intense. In other words, the intensity of light varies 

 inversely as the square of the distance from the luminous body. 

 These experiments may be varied in numerous ways with the 

 result of establishing the truth of this law. 



It is often desirable to compare the illuminating power of dif- 

 ferent flames. The art of doing this is called photometry. The 

 simplest method employed is to place the two flames to be com- 

 pared at such distances from a screen that the intensity of light 

 each throws upon it shall be equal. Suppose, for example, we 

 want to know how many times more light one candle will give 

 than another candle of inferior quality. Let a slender rod be 

 placed just in front of a screen, and then move the flames to 



