THE NATURE OF LIGHT. 375 



minating, one, four and nine square feet. One square foot at B will 



receive one-fourth, and one 

 square foot at C will receive 

 one-ninth as many rays as 

 one square foot at A. The 

 light being diffused over a 

 greater surface is corres- 

 pondingly diminished in in- 

 tensity. 



(6.) The experiment may 

 be tried by placing the large 

 screen at A and tracing the 

 outline of the shadow with 

 a pencil, then placing the 

 FIG. 277. screen successively at B and 



C, tracing the shadow each 



time. The experiment will be more satisfactory if a perforated 



screen be placed at 8. 



EXERCISES. 



1. A coin is held 5 feet from a wall and parallel to it. A lumi- 

 nous point, 15 inches from the coin, throws a shadow of it upon the 

 wall. How does the size of the shadow compare with that of the coin ? 



2. (a.) What is the velocity of light ? (6.) How was it determined ? 



3. (.) How are the intensities of two lights compared ? (&.) De- 

 fine light, (c.) Give your idea of the carrier of radiant heat and light. 



4. (a.} Define luminous, transparent, opaque, beam and pencil. 

 (ft.) How could you show that light ordinarily moves in straight 

 lines ? (c.) Explain the formation of inverted images in a dark room. 



5. (.) What are shadows ? (&.) By figures, illustrate shadows 

 when the intercepting body is greater, equal to and less than the 

 luminous body, and explain, (c.) What is the visual angle ? 



Recapitulation. In this section we have considered 

 the Nature of Light ; Luminous, Illuminated, 

 Transparent, Translucent and Opaque bodies ; 

 Rays, Beams and Pencils of light; that Light 

 Moves in Straight Lines; Inverted Images 

 and Shadows ; the Visual Angle ; the Veloc- 

 ity and Intensity of light. 



