414 PROCEEDINaS OF THE AMERICAN ACADEMY 



the labyrinth of the human ear. Whether we regard the sound, 

 therefore, at its origin, in its promulgation, 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 in seeing sound lies in the fact 

 that the acoustic vibrations are upon a microscopic scale of magnitude, 

 and, by their quick succession, the sepax-ate eiFects of individual vibra- 

 tions blend into one sensation, in the eye as well as in the ear, by vir- 

 tue 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 mirror attached to it, 

 which moves m 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 visible effect 

 which they originate, from a revolving mirror. By this device of 

 looking at the image of the body, instead of the body itself, its vibra- 

 tions, which coexist in space, are disentangled from each other, and 

 individual 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, Sa- 

 vart's contrivance for discovei'ing and exhibiting the nodal lines of 

 plates by means of sand sprinkled over their surface, the investigation 

 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's mirrors attached to tuning-forks, etc., Kcenig's 

 flames played upon by vibrating columns of air and reflected in a re- 

 volving mirror, and, finally, Melde's strings excited by the sympathetic 

 vibration of an attached tuning-fork or bell. 



The present communication is confined, however, to Koenig's re- 

 flected flames, in which are seen the individual vibrations of an organ- 

 pipe ; by which can be beautifully demonstrated to the eye : First, — 

 That the number of vibrations increases with the audible pitch ; Second, 



— That coexisting vibrations produce maxima and minima of motion 

 corresponding to the beats whicli are recognized by the ear ; Third, 



— That one column of air will respond, in sym|)athetic vibration, to 

 another, when there is an agreement between their fundamental notes 

 or some of their harmonics ; Fourth, That two unison-pipes, brought 



