POLYTECENIC ASSOCIATION PROCEEDINGS. YQ'Q 



I. ON THE OPTICAL METHOD OF STUDYING SOUND. 

 By Joseph Lovering, of Cambridge, Mass. 



When the science of acoustics is studied by means of tne ear 

 exclusively, we judge of the process simply by the result, that is, 

 by the sensation. The optical method of investigation often gives 

 us an insight into the process itself. Sound begins with a stationaiy 

 vibration in the sonorous body; it is propagated by a progressive 

 undulation; and it ends, physically and mechanically considered, 

 in a vibration of some one of the three thousand nervous fila- 

 ments discovered by Corti in the labyrinth of the human ear. 

 Whether we regard the sound, therefore, at its origin, in its pro- 

 mulgation, or in the sensation, it is nothing but a vibration; and 

 vibration is motion, and motion is the subject of vision. So that 

 to see sound is only to see the motions which cause it. The only 

 difficulty of seeing sound lies in the fact that the acoustic vibra- 

 tions are upon a microscopic scale of magnitude, and, by their quick 

 succession, the separate effects of individual vibrations blend into 

 one sensation, in the eye as well as in the ear, by virtue of what is 

 called in both cases the persistency of the impression on the organ 

 of sensation. To overcome the first difficulty, a beam of light is 

 reflected from the vibrating body, or a muTor attached to it, which 

 moves in angle twice as fast as the body itself, while the motion in 

 arc may be amplified to any extent by increasing the length of the 

 beam of light. The second difficulty is surmounted by reflecting 

 the vibrations of the sonorous body itself, or some more visiblo 

 effect which they ori inate, from a revolving mirror. By thifl 

 device of looking at the image of the body, instead of the bodj- 

 itself, its vibrations, which coexist in space, are disentangled from 

 each other, and movement vibrations, hundreds of which succeed 

 each other in a single second of time, are translated into a long 

 belt of space, in which even two successive ones do not overlap. 



The optical method of studying sound embraces, in general. 

 Savart's contrivance for discovering and exhibiting the nodal lines 

 of plates by means of sand sprinkled over their surface, the inves- 

 tigation of the nodes and bellies of sounding strings by mounted 

 riders, and of columns of air by a little drumhead suspended in 

 the pipes, and, more recently, Lissajous' mirrors attached to tuning- 

 forks, etc., Koenig's flames played upon by vibrating columns of 

 air and reflected in a revolving mirror, and, finally, Melde's strings 

 excited by the sympathetic vibration of an attached tuning-fork 

 or bell. 



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