242 



PHYSIOLOGY 



CHAP. 



developments of the method will make it possible to represent the 

 lowest and highest perceptible notes in specific sound-images. 



Ewald's acoustic images can be photographed like any other 

 microscopic image, since they consist of a series of stationary 

 waves. Fig. 97 shows a specimen of these photographs ; it repre- 

 sents the most central part of the series of waves that extend 

 over the whole length of the membrane. As the waves can only 

 be seen and photographed when the membrane is viewed obliquely, 



the regular form of the wave is obtained 

 only in the centre of the figure, which 

 corresponds to the focus of the micro- 

 scope. 



The acoustic images are obtained not 

 only when the different musical notes 

 are produced by a Galton whistle near the 

 membrane, but also when it is immersed 

 in water, under conditions which schema- 

 tically reproduce those of the inner ear. 

 c Ewald constructed a camera acoustica 

 which represents a model of the auditory 

 apparatus, as G. B. Porta's camera obscura 

 is a model of the visual apparatus (Figs. 

 98, 99). 



" m 



FIG. 97. Photograph of the auditory 

 image of a tone obtained with the 

 membrane shown in Fig. 95. 



FIG. 98. Ewald's camera acoustica. 

 Explanation in text. 



Ewald constructs the elastic membrane for the reproduction of the 

 stationary waves as follows. In an aluminium disc 0'075 mm. thick he cuts 

 a rectangular slit with sharp, smooth edges (Fig. 95). He then plunges the 

 disc by means of forceps into a solution of india-rubber and benzene (1 grm. 

 non-vulcanised rubber in 20 cc. benzene) and withdraws it rapidly. After 

 removing the excess solution from the edges of the disc he dries the thin 

 layer of fluid which covers the slit in the disc, by waving it gently in the 

 air, taking care that the delicate membrane is of uniform thickness through- 

 out. It is only under these conditions that the membrane can be utilised 

 for observation of Ewald's stationary waves. 



His camera acoustica consists of a chamber, entirely filled with water, and 

 divided inside by a partition (c of Fig. 98) which can be drawn out, and 

 holds the capsule e in which is the aluminium disc with the elastic membrane. 

 The stationary waves produced upon this membrane are observed in the 

 microscope, illuminated through the glass walls of the chamber. 



