"A pulsatory current results from sudden or instan- 

 taneous changes in the intensity of a continuous current ; 

 and 



"An undulatory current is a current of electricity, the 

 intensity of which varies in a manner proportional to 

 the velocity of the motion of a particle of air during the 

 production of a sound; thus, the curve representing 

 graphically the undulatory current for a simple musical 

 tone is the curve expressive of a simple pendulous vi- 

 bration that is, a sinusoidal curve 



"I have before alluded to the invention by my father 

 of a system of physiological symbols for representing 

 the action of the vocal organs, and I had been invited 

 by the Boston Board of Education to conduct a series 

 of experiments with the system in the Boston school 

 for the deaf and dumb. It is well known that deaf- 

 mutes are dumb because they are deaf, and that there 

 is no defect in their vocal organs to incapacitate them 

 from utterance. Hence it was thought that my father's 

 system of pictorial symbols, popularly known as visible 

 speech, might prove a means whereby we could teach the 

 deaf and dumb to use their vocal organs and to speak. 

 The great success of these experiments urged upon me 

 the advisability of devising methods of exhibiting the 

 vibrations of sound optically, for use in teaching the 

 deaf and dumb. For some time I carried on experi- 

 ments with the manometric capsule of Koenig, and 

 with the phonautograph of Leon Scott. The scientific 

 apparatus in the Institute of Technology in Boston was 

 freely placed at my disposal for these experiments, and 

 it happened at that time a student of the Institute of 



