154 LIGHT AND SOUND 



The area of the smaller card will be 1 square inch; of the larger, 4 

 square inches. If we place the smaller card 4 inches from the lamp, 

 and the larger card 8 inches (twice as far) from the lamp, then the 

 shadow cast by the first card will just cover the second card. This 

 means that the light which falls on 1 square inch at distance 1 is 

 spread out over 4 square inches at distance 2. Therefore the intensity, 

 or brightness, of the light on the more distant card is not K of the 

 brightness on the nearer card, but 14 X K, or 34 as great. 



171. Candle Power. The intensity of the light given 

 off by lamps is always stated as so many " candle power." 

 This is because the unit of intensity is the illuminating 

 power of a standard candle. The standard candle is of 

 sperm fat, weighs Ye of a pound, and burns at the rate of 

 120 grains (7.78 grams) an hour. 



We get the candle power of a lamp 

 by using a photometer (Fig. 145). This 

 consists of a piece of oiled paper or 

 cardboard, supported in a frame, and a 

 pencil that can be set upright near the 

 frame. The work must be done in a 

 FIG. 145. room that is dark, except for the lamp 



How to get the " candle power " of an d can dle to be compared. These two 



an electric light. r 



are lighted, and placed so that the two 



shadows of the pencil shall fall on the oiled screen side by side. The 

 distance of the lamp or candle is then altered, so that the shadows are 

 equally dark. Suppose that the distance between the candle and its 

 shadow is 1 foot, and that between the lamp and its shadow, 4 feet. 

 We square each of these numbers; that is, we multiply each by itself. 

 The square of 1 is 1, and that of 4 is 16. So the lamp has an illuminat- 

 ing power 16 times as great as that of the candle. We say it is a " 16 

 candle-power " lamp. 



172. Exercises. 



1. At about 525 C. iron becomes red hot. Is iron illuminated 



