MONEY VALUE OF LIGHT 



107 



electric light of a standard strength, and naturally full value 

 is desired. An instrument for measuring the strength of 

 a light is called a photometer, and there are many different 

 varieties, just as there are varieties of scales which measure 

 household articles. One light-measuring scale depends upon 

 the law that the intensity of illumination decreases with the 

 square of the distance of the object from the light. Sup- 

 pose we wish to measure the strength of the electric light 

 bulbs in our homes, in order to see whether we are getting 

 the specified illumination. In front of a screen place a black 

 rod (Fig. 58) which is illuminated by two different lights; 

 namely, a standard 

 candle and an incan- 

 descent bulb whose 

 strength is to be meas- 

 ured. Two shadows of 

 the rod will fall on the 

 screen, one caused by 

 the candle and the other 

 caused by the incandescent light. The shadow due to the latter 

 source is not so dark as that due to the candle. .Now let the 

 incandescent light be moved away from the screen until the 

 two shadows are of equal brightness or darkness. If the in- 

 candescent light is four times as far away from the screen as 

 the candle, and the shadows are equal, we know, by Section 

 100, that its strength is sixteen candle power. If the incan- 

 descent light is four times as far away from the screen as the 

 candle is, its power must be sixteen times as great, and we 

 know the company is furnishing the standard amount of light 

 for a sixteen candle power electric bulb. If, however, the 

 bulb must be moved nearer in order that the two shadows 

 may be of equal brightness, the power of illumination is less 

 than the standard contract. 



FIG. 58. The two shadows are equally dark. 



