PHYSICAL CIIARACTERISIICS OF AUDITION 63 



countered in fittinj^ the eyes with jjlasses. It is found that people 

 with two eyes which are slightly different do not see stereoscopically 

 but if glasses are made so as to compensate and make the eyes nearly 

 alike, it usually takes a certain amount of practice before the sense 

 of perspective can be brought back. 



APPENDIX 



9. Experimetital Methods. In order to discuss the principles of ear 

 sensitivity measurement on a rigorous dynamical basis, it will perhaps, 

 be clearer to describe briefly the experimental method used in pro- 

 ducing known sound pressure in the ear canal at the various fre- 

 quencies and intensities.'* 



As a source of sound, a small thermal receiver unit was used. This 

 consisted ot about twenty very small loops of Wollaston wire con- 

 tained in a brass case small enough to be inserted in the external ear 

 canal and entirely stop it up. In the average ear a volume of about 

 1 cm.^ of air is included between it and the drum membrane. A direct 

 heating current is passed through the receiver and an alternating 

 current of the desired frequency and intensity is superimposed to 

 modulate its temperature. This modulation in temperature causes 

 alternate expansion and contraction of a very thin film of air covering 

 its surface and so produces alternations in pressure in the ear canal 

 of the frequency of the impressed alternating current. The intensity 

 is proportional to this alternating current if it is maintained small 

 compared with the direct current. This arrangement permits of 

 producing alternating or sound pressure on the ear drum with a 

 comparatively simple dynamical relation between the source of 

 sound and the ear drum. The thermal receiver is also dynamically 

 one of the simplest sources of sound known. 



The sound or alternating pressure was determined by calibration. 

 This was done by inserting the thermal receiver in an air cavity of 

 1 cm.^ volume in front of a condenser transmitter diaphragm by which 

 the alternating pressure developed by a given current in the receiver 

 could be measured.^ By measurement of the current for minimum 

 audibility or "maximum audibility" or for any other intensity the 

 pressure in the ear canal is determined. 



10. Dynamical Principles of Ear Measurements. From a dynamical 

 standpoint the phrase "sensitivity of the ear" as it is usually used is 



* For further details, see "The Frequency Sensitivity of Normal Ears," H. Fletcher 

 and R. L. Wegel, Physical Review, June, 1922. 



* For the method of calibration of the condenser transmitter, see article by H. D. 

 Arnold and I. B. Crandail, Physical Review, July, 1917. 



