PHOTOGRAPHY OF SOUND WAVES. 359 



from the point; the same is true in the case of the echo, the rays 

 radiating from the image point below the reflecting surface. In all 

 subsequent cases the reader can, if interested in tracing the analogy 

 between sound and light, draw lines perpendicular to the reflected wave 

 surfaces representing the system of reflected waves. 



We will now consider a second case of reflection. We know that if 

 a lamp is placed in the focus of a concave mirror, the rays, instead of 

 diverging in all directions, issue from the mirror in a narrow beam. 

 The headlight of a locomotive and the naval searchlight are examples 

 of the practical use made of this property. If the curvature of the 

 mirror is parabolical, the rays leaving it are parallel; consequently 

 mirrors of this form are employed rather than spherical ones. But what 

 has the mirror done to the wave surface which is obviously spherical 

 when it leaves the lamp, and what is its form after reflection? The 

 wave surface, I have said, is always perpendicular to the rays: conse- 

 quently in cases where we have parallel rays we should expect the wave 

 to be flat or plane. 



Examine the second photograph, which shows a spherical sound wave 



Fig. 2. Spheric at. Sound Wave. 



starting at the focus of a parabolic mirror. The echo appears as a 

 straight line, instead of a circle as in the previous case, which shows 

 us that the wave surface is flat. 



If now our mirror is a portion of a sphere instead of a paraboloid, 

 our reflected wave is not flat, and the reflected rays are not all parallel, 

 the departure from parallelism increasing as we consider rays reflected 

 from points farther and farther away from the center of the mirror. 

 A photograph illustrating the reflection of sound under these conditions 

 is next shown, the echo wave being shaped like a flat-bottomed saucer. 

 As the saucer moves upward the curved sides converge to a focus at the 

 edge of the flat bottom, disappearing for the moment (as is shown in 

 the fourth picture of the series), and then reappearing on the under 

 side after passing through the focus, the saucer turning inside out. 



If, instead of having a hemisphere, as in the last case, we have a 

 complete spherical mirror, shutting the wave up inside a hollow ball, 

 we get exceedingly curious forms; for the wave can not get out, and is 

 bounced back and forth, becoming more and more complicated at each 

 reflection. This is illustrated in our next photograph, the mirror being 



