SL'iNSKS 



resting upon it. This last is the essential stimulating act, while all that pre- 

 cedes is more or less accessory or contributory to this act. Just what the 

 accessory apparatus contributes can best be understood by an examination 

 of the stimulus and the sensation which results from its action. 



Sound. Any elastic body, e.g., air, a membrane, or a string, per- 

 forming a certain number of regular vibrations per second, gives rise to 

 what is termed a musical sound or tone. We must, however, distinguish 

 between a musical sound and a mere noise; the latter being due to irregular 

 vibrations. 



Musical sounds are distinguished from each other by three qualities: 

 i, Strength or Intensity, which is due to the amplitude or length of the wave 

 of vibrations. 2, Rate, the number of vibrations in a second. 3, Quality, 

 or Timbre, the peculiar property by which we distinguish the same note 

 sounded on two instruments, e.g., a piano and a flute. It has been proved 

 by Helmholtz to depend on the number of secondary tones, termed harmonics, 

 which are present with the predominating or fundamental tone. That is, 

 rhythmic vibrations are either simple in form, like the vibrations of a reed 

 or tuning fork, or compound, like the vibrations of a violin or piano string. 

 If the string of a violin is plucked it not only vibrates as a whole, but in seg- 

 ments in the ratio of one, two, three, etc. The form of air wave that is pro- 

 duced by several such vibrating bodies is very complex indeed, as, for example, 

 when an orchestra is playing. 



The compound wave can be analyzed into its constituent elements by a 

 system of resonators, on the principle of sympathetic vibration. If one 

 sounds a series of musical notes before such a system of resonators it will be 

 found that the tones and overtones are selected by the resonators and made 

 more prominent so that they can be identified. 



The sensation of sound has in it certain elements that correspond closely 

 with the physical properties of sound, i.e., loudness, pitch, and quality. Loud- 

 ness is dependent merely on the intensity of the stimulation. A sound wave 

 of great energy, for example, produces a larger movement of the tympanic 

 membrane, and it, through the chain of bones and the fluid of the internal 

 ear, a larger swing of the basilar membrane, hence a more intense stimulus 

 of the organ of Corti. 



Function of the External and Middle Ears. It has already been 

 stated that the external ear collects the sound waves and conducts them 

 against the membrana tympani. This membrane vibrates as a whole to 

 the compound waves that impinge upon it, and thus serves for the trans- 

 mission of sound from the air to the chain of ossicles of the middle ear. It is 

 often compared to the membrane of a drum, but there are fundamental 

 differences. 



When a drum is struck, a certain definite fundamental tone is elicited; 

 similarly a drum is thrown into vibration when certain tones are sounded in 



