LIGHT. 297 



the space behind sucli a body. The comparative dark- 

 ness thus produced is called a shadow. 



■When tlie light-giving surface is greater than the body casting the shadow. 

 -i^cross section of the shadow thrown upon a plane surface will be less than 

 the body ; and less, moreover, the further this surface is from the body, for 

 the shadowed space terminates in a point. 



TrVTien the luminous center is smaller than the opaque body casting the 

 shadow, the shadow will gradually increase in size \A'ith the distance, without 

 limit ; thus the shadow of a hand held near a candle, and between a candla 

 and the wall, is gigantic. 



If the shadow of any object bo thrown on a wall, the closer 

 circumstances the opaque bod}' is held to the light-producing center, as a 



will thetize of candle, for example, the larger will be its shadow. The rea- 



a shadow be ' ^ ' ° 



incrpafted or son of this is, that the rays of light diverge from the center 



dimims ed ^ straight lines, Uke lines drawn from the center of a circle ; 



and therefore the nearer the object 

 ■ * ■ is held to the center, the greater 



the number of rays it intercepts. 

 Thus, in Fig. 231, the arrow A, held 

 close to the candle, intercepts a large 

 number of rays, and produces the 

 shadow B F; while the same ar- 

 row held at C, intercepts a smaller 

 number of rays, and produces only 

 the little shadow D E. 



"W'heu two or more luminous ob- 

 — ^ ' 



■~~-.^ i jccts, not in the same straight line, 



'~~-J shine upon the same object, each one 



^ will produce a shadow. 



646. The intensity of li^ht which issues 



How does the .,...,.■■ 



intensity of from a luminous point diminishes in the same 



light vary? . •■■ pit /• 



proportion as the square oi the distance irom 

 the luminary increases. 



Thus, at a distance of two feet, the intensity of light will be one fourth of 

 "what it is at one foot ; at three feet the intensity will be one ninth of what it 

 is at one foot. In other words, the amount of illumination at the distance of 

 one foot from a single candle would be the same as that from four, or nina 

 candles at a distance of two or three feet, the numbers four and nine being 

 the squares of the distances two, and three, from the center of illumination. 

 Upon what ^^^- ^his law, therefore, may be made available for meas- 



principle may urmg the relative intensities of light proceeding from different 

 tensities of sourccs. Thus, in order to ascertain the relative quantities of 

 different lu- jigij^ furnished by two different candles, as, for example, a 

 be ascertained? wax and a tallow caudle, placa two discs or sheets of whit« 



13* 



