1870.] Light and Sound. 7 



ance of a landscape, it would appear almost inconceivable how such 

 varied impressions could be individually conveyed to the mind. 

 This intricate problem is solved in the most exquisite manner; 

 moreover, in a manner that appears to be essentially the same in 

 the case of the eye and the ear. 



Let us approach this analogy with an illustration. When a 

 piano is opened an attentive listener will readily discover that every 

 time a note is sung in the room, that string of the piano which 

 would have yielded the same note is thrown into sympathetic 

 vibration. If the note be changed, another and corresponding 

 string responds. Imagine now that a deaf man has his fingers 

 lightly touching the strings of the piano ; he will perceive when 

 they vibrate and which vibrates. He will thus become conscious 

 what note has been sung. A similar arrangement to the foregoing 

 is fitted within each of our ears. In the inner ear there is a con- 

 trivance known as Corti's organ, which consists of no less than 

 3000 differently strained fibres. These fibres constitute the out- 

 ward extremity of the auditory nerve : a vibration of any one of 

 them immediately travels to the brain. According to one of the 

 greatest living investigators, Helmholtz, it is believed that one or 

 more of the fibres enters into sympathetic vibration whenever a 

 sound reaches the ear. Corti's organ is to us, therefore, merely 

 a refinement and extension of what the piano was to the deaf man. 

 M. Hensen's experiments upon the means of hearing in the Crustacea 

 confirm this view. 



As, however, it would be impossible for the strings of a piano 

 to vibrate if the note were beyond the range of the instrument in 

 either direction, so also there is a limit to the perception of sound 

 by the ear. If the aerial waves recur more quickly than 38,000 

 times a second, no sound at all is heard, however intense the vibra- 

 tion. If the waves recur slower than sixteen times a second, they 

 are inaudible as a continuous sound. Hence the extreme range 

 of our hearing embraces about eleven octaves. This limit varies 

 slightly in different persons, as first shown by Dr. Wollaston.* For 

 example, some persons whose hearing is otherwise good cannot hear 

 the chirp of a cricket or the squeak of a bat. Moreover, one ear 

 is often more sensitive than the other,t and some ears are more 

 sensitive to one class of sounds than to others. 



Now, with regard to vision, a similar arrangement to that in the 

 ear appears to exist at the back of the eye. The optic nerve there 



* « Phil. Trans.,' 1820, p. 306. 



f When travelling in Norway last year the writer observed that on one occa- 

 sion the sounds from myriads of grasshoppers were heard, but on closing the 

 left ear there was perfect silence : opening the left ear and closing the right ear, 

 the sound was heard as loud as ever. The difference, where it exists, may readily 

 be noticed by listening to the ticking of a watch. The cause, probably, arista 

 from habitually sleeping on the right side. 



