GRAPHIC RECORDING OF SPEECH VIBRATIONS 
93 
yond the mirror M, which prolongs the focus to on the screen 
P. Other factors are as in form A. It may be found advisable 
also to try two glass-tube mirrors instead of one. For the pos- 
sible arrangement in that case, compare the place of the two 
mirrors in form B of the sonograph in figure' 5. 
In form C (fig. 3), a concave lens L 2 replaces the mirror M 
of form B. Other details are as in form B. The lens L 2 would 
have to be very small and might therefore be more difficult to 
obtain and adjust than the glass-tube mirror. Form C there- 
fore seems to offer less promise of success than forms A and B. 
F 
Fig. 3. Principle of the Sonoscope, Form C (Horizontal Section) 
The sonograph. The principle of the sonograph is shown in 
figures 4, 5 and 6. It consists of a sound-proof cylinder SC and 
S'C' completely cut across by an oblique opening 00 comprised 
between two parallel planes. A ray of light R passing throug 
the aperture A and the collecting lens L], all in the first section 
of the sound-proof cylinder SC, thence passes through a paral- 
lelogram prism Pti into the opening 00, through a second prism 
Pr ’2 into the second sound-proof section S'C', where it passes 
through diverging lenses or mirrors to the screen or film PP. 
The ray R is refracted away from the perpendicular as it passes 
