W. LeConte Stevens— Projection of Acoustic Curves. 235 
rest; a vertical band is seen, longer than either of the previous 
ones, with spots of increased brightness not only at the ends 
ut also at intermediate points, the beam of light being mo- 
mentarily arrested at several intervals in each compound 
vibration (fig. 1, EF). -On revolving the mirror, a series of 
compound curves appears on the screen, one of which is repre- 
sented by the continuous curve in fig. 1. In this the two forks 
are supposed to begin exactly together. If there be any 
other combination of phases than this, the result is a corre- 
sponding modification in the form of the curve. Practically, 
y loading either fork very slightly, the curve may be changed 
through all its modifications within a few seconds. 
Optical Presentation of a Discord. 
Select a pair of unisonant forks, and arrange as already 
described. Toad one of them until any desired number of 
utror is at rest. The vertical band of light grows alternately 
long and short, with the swelling and softening of the sound, 
Revolve the mirror rather slowly but uniformly ; the sinuous 
curve broadens vertically at every beat and is narrowed 
at every approximate silence (fig. 2). By using forks of mod- 
sttely high pitch and obtaining six or eight beats per second, 
'scord may be seen and heard at the same time. 
. This method of exhibiting the composition of musical sounds 
® i some respects preferable to the method of manomet 
ny the effect of difference of initial phase in determining 
7 duality of a combination, as emphasized by Konig. 
@ 
familiar. Tyndall,+ Blasernat and others, have described a 
* Konig, Acoustique, pp. 222-243, Paris, 1882. 
no. Sound, pp. 366-369, Appleton, 1877. as 
Blaserna, Sound and Music, pp. 150-152, Appleton, 1876. 
