THE SPECIAL SENSES. 



84. Old-sight, or Presbyopia. But this marvel- 

 lously beautiful mechanism becomes worn with use; or, 

 more strictly speaking, the lens, like other structures of the 

 body, becomes harder with the approach of old age. The 

 material composing the lens becomes less elastic, the power 

 to increase its curvature is gradually lost, and as a conse- 

 quence, the person is obliged to hold the book further away 

 when reading, and to seek a stronger light. In a word, the 

 function of accommodation begins to fail, and is about the 

 first evidence that marks the decline of life. By looking at 

 the last preceding diagram, and remembering that the in- 

 creased curvature of the lens cannot take place, it will be 

 at once understood why old-sight is benefitted in near 

 vision by the convex lens, such as the spectacles of old 

 people contain. It acts as a substitute for the deficiency 

 of the crystalline lens. 



85. The Sense of Hearing. Sound. Hearing, or 

 audition, is the special sense by means of which we are 

 made acquainted with sound. What is sound? It is an 

 impression made upon the organs of hearing, by the vibra- 

 tions of elastic bodies. This impression is commonly prop- 

 agated by means of the air, which is thrown into delicate 

 undulations, in all directions from, the vibrating sub- 

 stance. When a stone is thrown into smooth water, a wave 

 of circular form is set in motion, from the point where the 

 stone struck, which constantly increases in size and dimin- 

 ishes in force, as it advances. 



86. Somewhat resembling this, is the undulation, or 

 sound-wave, which is imparted by a sonorous vibration to 

 the surrounding atmosphere. Its shape, however, is spher- 

 ical, rather than circular, since it radiates upward, down- 

 ward, and obliquely as well as horizontally, like the wave 



84. Change of sight with the approach of old age ? Explain the change ? 



85. Hearing or audition? What is sound? How propagated commonly.' 

 Stone thrown in water? 



86. Sound-wave in the atmosphere? Its shape? Rate of motion ? Sound, in 

 water, air, and solid bodies ? 



