934 W. LeConte Stevens—Projection of Acoustic Curves. 
ArT. XXXI.— Optical Projection of Acoustic Curves; by 
W. LEeContEe STEVENS. 
Tue following application of a well known principle may 
be of sufficient interest to teachers of Physics to justify its 
publication; it has not been elsewhere described, so far as 1s 
known to the writer. 
appropriate measurements on paper the sinusoids repre- 
senting the tones composing any group of musical sounds can 
be compounded, and the resultant curve represents the con- 
cord or discord produced. The construction of such curves 
involves much tedious work. ny of them may be easily 
produced and optically presented on a screen by a simple 
modification of the method commonly employed in projecting 
Lissajous’ curves. 
Optical Presentation of a Concord. 
Select a pair of tuning forks provided with mirrors and 
sounding tones whose vibration frequencies bear to each other 
such a simple ratio as 2:1 or 3:2. Let us suppose the tones 
selected to be C and G, with sixty-four and ninety-six vibra- 
tions per second respectively. Arrange the apparatus as for 
Lissajous’ experiment, except that both forks shall vibrate 10 
vertical planes. t any convenient distance in front of the 
fork from which the second reflection of light occurs, place 4 
mirror capable of revolution about a vertical axis, such as Is 
used with the manometric flame. From this the spot of light 
is focussed upon a screen. 
Excite the G-fork; the spot is lengthened into a narrow 
vertical band, faint in the middle and a at the ends, as 
approximately shown in ; ‘ urn the mirror; 
s oe mprkanme SC 
fig. 1. 
e 
8 spread into sinusoids, two of which are represent 
i . 
long-dotted curve. Excite both forks while the mirror 1s at 
