July 8, 1922] 



NA TURE 



In 1890 I proposed to use a diaphragm made of 

 paper, which should be placed, shielded on one side, at 

 the point where the sound was to be measured. In 

 order that the effect of the sound should not be dis- 

 torted, the membrane, instead of having to do any 

 work, as in the case of the diaphragm of the phono- 

 graph in digging up the wax, or in that of the micro- 



mitted 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 intro- 

 duced by the use of a thin plate of mica for the 

 diaphragm. 



To obtain the sensitiveness necessary to measure 

 sounds of ordinary intensity, the property of resonance 



phone in compressing the carbon, was to be perfectly 

 free, but was to carry a small plane mirror cemented 

 on at its centre. In close juxtaposition and parallel 

 with this was the 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 



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 



Parts of the phonometer. 



sound falls upon the diaphragm the fringes vibrate 

 rapidly and disappear from sight. 



By the introduction of a Michelson optical interfero- 

 meter, 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 per- 



NO. 2749, VOL. IIO] 



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 



