SUMMARY 171 



ments of the lines of force of the magnet N, and produce 

 vibrations in the disk E. The vibrations of E are just 

 like those of Z), and reproduce the sound we made at the 

 other end of the line. 



In the modern telephone the " return circuit " is 

 through a second wire 

 instead of through the 

 earth. The receiver is 

 like the simple disk of 

 the figure, but the speak- 

 ing instrument ("trans- 

 mitter") is more complex. 

 The telephone was in- 

 vented in 1875 by Alexander Graham Bell, of Washington, 

 and Elisha Gray, of Chicago. 



194. Summary. A visible body is either self-luminous or illuminated. 



Bodies may be transparent, translucent, or opaque. 



Light is the cause that produces sight. It moves in straight lines at 

 a speed of 186,000 miles a second. 



An image formed through a small opening is inverted. 



A shadow formed from a point of light is distinct, and composed of 

 an umbra alone. A shadow from a luminous surface has both umbra 

 and penumbra. 



An eclipse of the sun occurs when the earth is within the moon's 

 shadow. An eclipse of the moon occurs when the moon is in the 

 earth's shadow. 



Intensity of light is given in candle power. A photometer is an in- 

 strument for finding when the intensity of light from two luminous 

 bodies is the same. 



Light striking a body may be reflected, dispersed, or absorbed; or 

 it may pass through the body. 



Light is reflected at the same angle as that at which it strikes the 

 reflecting surface, but in a different direction. 



