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Mr. A. G. Webster 



[June 10, 



plane side of a lens which, viewed in the light from a sodium flame, 

 was to give Newton's rings or interference fringes. Of course, when 

 the sound falls upon the diaphragm the fringes vibrate rapidly and 

 disappear from sight. 



By the introduction of a Michelson optical interferometer, two of 

 the difficulties of this instrument were overcome — namely, (1) that of 

 adjusting the lens so that it would not strike the vibrating mirror, 

 since the mirrors in the interferometer could be as far apart as one 

 pleased : and (2), more important still, it permitted the use of fringes 

 in white light, so that it was possible to use gas, incandescent, or arc 

 light with excellent effect. A further improvement was introduced 

 by the use of a thin plate of mica for the diaphragm. 



Fig. 1. — Phonometer. (Interferometer not shown.) 



To obtain the sensitiveness necessary to measure sounds of 

 ordinary intensity, the property of resonance is employed twice— i.e. a 

 system of two degrees of freedom is used. First, the plate resounds 

 to a sound more strongly as it is tuned more nearly to it ; and second, 

 a resonator that can also be tuned is put behind the plate. The 

 sound entering by the hole in the resonator is magnified by the 

 tuning, and acts upon the plate, which is also tuned. A graph can 

 be plotted in which one co-ordinate represents the stiffness of the 

 plate, or rather what may be called the mistuning, which is the 

 stiffness lessened by the product of the mass by the square of the 

 frequency. The other co-ordinate represents the corresponding 

 quantity for the resonator, the stiffness of which depends simply on 

 the volume into which the air is compressed, while the effective mass 

 depends on the dimensions of the whole, and its damping on the 

 sound radiated from the mouth. It is then found that the tuning 



