HOW SOUNDS ARE MADE AND CARRIED 167 



the negative (Fig. 161). In it the light parts of the object are dark 

 (opaque), and the dark parts are light. The photographer " prints " 

 the positive picture by placing the negative over sensitive paper con- 

 taining silver chloride, and exposing the two for some time to sunlight. 

 The paper will then be changed as the negative was. But as the dark 

 parts of the negative cut off the light, while the lighter parts permit 



FIG. 161. 

 Negative and Positive Photograph. Negative by H. L. Schall. 



it to pass through, the positive "print" contains light and shade in 

 the same relation as the object photographed. 



189. How Sounds are Made and Carried. Just as light 

 is the cause that affects the optic nerve and gives us sight, 

 so sound is the cause that affects the nerve of the ear and 

 causes hearing. All sound is due to the motion of some 

 portion of matter. When a violin string is producing a 

 note, it is moving rapidly back and forth. We can prove 

 this by touching the string lightly with the finger. We 

 can prove the same thing in the case of a sounding tele- 

 phone or door bell. When we speak or sing, we force air 

 through the slit between the vocal cords (cf. 389), and 

 set them into rapid motion. When a vibrating tuning 



