422 PHYSIOLOGICAL PHYSICS. [Chap. xxxi. 



conversation in a low voice through an iron tube 

 3,120 feet long. The intensity of a sound increases 

 also with the density of the medium in which it is 

 produced. Such principles explain to some extent the 

 action of the otoscope and stethoscope. The former 

 consists of a narrow iiidiarubber tube with ivory 

 ends. One end is placed in the canal of the ob- 

 server's ear, and the other in relation to the sounding 

 body, and thus the sound is conveyed to the observer's 

 ear without enfeeblement. The stethoscope acts not 

 only by its own particles conducting the sound, but 

 because the tube prevents the dissipation of the vibra- 

 tions. 



A stethoscope, bearing some resemblance to a 

 Marey's tambour, was devised by Koenig in 1864. 

 It consists of a metallic ring, oil each side of 

 which is attached a thin indiarubber membrane, so 

 disposed that when the enclosed space is inflated 

 through an opening in the ring the chamber assumes 

 the form of a bi-convex lens. A stopcock shuts the 

 chamber oft' from the outside. Connected with the 

 edge of the metallic ring is a hemisphere of metal 

 which covers in one of the indiarubber membranes, a 

 space intervening between the two. In the middle of 

 the hemisphere is a tube from which an indiarubber 

 tube, terminating in an ivory end, is carried to the 

 ear. The exposed indiarubber wall is applied to the 

 body to be examined. Vibrations are communicated 

 to the air in the indiarubber chamber, and from it to 

 the air in the space between the metal hemisphere and 

 the indiarubber chamber, and so along the tube to the 

 ear. Several tubes may be led from the hemisphere, 

 and several persons may hear at the same time. 



A reflection of sound is effected by the auricular 

 appendages of the ears. It has been shown that their 

 form is peculiarly adapted for concentrating the 

 sound waves into the external canal by which they 



