346 



THE SENSES 



feet) per second), and consist in alternate rarefactions and compressions 

 of the air. The vibrations move back and forth in the line of advance. 



FIG. 197 



FIG. 198 



\ 





A suggestion as to the nature of sound-vibrations as distinct from those producing light 

 (which are vertical to the ray's direction). Moreover, the sonorous vibrations are those of 

 material particles of air, while those of light are of the ether, perhaps not of a material nature. 



(while in case of light, the vibrations are at right angles to the line of 

 advance). For the range of the human ear these waves vary in length,, 

 that is from crest to crest, from about 1.5 cm. to more than 12 m.; 



their number varies from about 16 to 

 35,000 (even 50,000?) per second. In case 

 of noises, all sounds which have not the 

 qualities of musical tones, the conditions 

 are more complex and various and in fact 

 little understood. It is supposed that the 

 vibrations producing them are irregular 

 in many ways, while those making tones 

 are essentially rhythmic however complex 

 the rhythm. It is presumed by many 

 that it is the ampullae of the semicircular 

 that represent noise, these being homolo- 

 gous to the hearing-organs of many of the 

 molluscs and other simple animals. (See 

 Figs. 221 and 222.) 



As in case of vision, we may divide the 

 ear for descriptive physiological purposes 

 into a transmitting part and a receiving 

 part. The conditions are relatively sim- 

 ple until we come to the essential nerve 

 end-organ of Corti. Even this we can 

 sketch in merest outline only, for this 



structure, minute as it is, makes the ear perhaps the most complicated of 

 all known mechanisms. At any rate it has a complexity, even that which 

 is known, far beyond the power of man at present to understand. 



Diagram to suggest the compound- 

 ing of sound-waves out of simpler 

 forms. In this case the bottom curve 

 (4) represents the combination of the 

 other three curves, these beating in 

 the ratio of 1, 2, and 3. (McGregor 

 Robinson.) 



