I9IS-] 



SURFACES OF TELEPHONIC DIAPHRAGMS. 



101 



patches, either quiescent, or with beats. By means of the optical 

 magnification of amphtude that can be efifected with such an ex- 

 ploring mirror and scale, diaphragm vibrations of amplitude o.i^ 

 (i. e., lO"^ cm.), or less, can be observed; although the precision of 

 measurement falls off considerably, for a diaphragm amplitude 

 below 0.5 /x (half a micron). 



Optical System. 



The optical system employed with the vibration explorer is dia- 

 grammatically indicated in Fig. 5. The stereopticon arc-lamp A 

 throws a powerful condensed beam of light on the pinhole B, in a 

 brass vertical screen. A set of small powerful collimating lenses C 



Diaphragm 



^Screen 



Fig. 5. Diagram of Optical System used with Vibration Explorer. 



throws the nearly paralleled beam through the screen and slit D, as 

 well as the focussing lens H, on the exploring mirror E^ whence 

 it is reflected to the translucent screen F, at a convenient distance, 

 in this case 25 cm. An image of the slit in screen D is then sharply 

 focused at F. In Fig. 6, it is indicated geometrically that the ampli- 

 tude ^ of the diaphragm's displacement is equal to the continued 

 product of the observed amplitude d of the luminous band, the ratio 

 of / (the radius arm of the mirror), to 2L the double distance of 

 the mirror from the screen, and the cosine of the angle ^ between 

 the radius arm of the mirror and the plane of the diaphragm. In 

 order to avoid frequent changes in (/>, it is desirable to keep con- 

 stant the zero of the spot at the center of the graduated scale F, 

 and with it the contacting angle of the mirror. The numerical 



