MOMENTARY DEAFNESS. 287 



appears to shine with a nebulous undefined light. This 

 is to the eye what such a hollow sound is to the ear.''* 



It has been recently shown by M. Savart, that the 

 human ear is so extremely sensible as to be capable of 

 appreciating sounds which arise from about twenty-four 

 thousand vibrations in a second, and consequently, that it 

 can hear a sound which lasts only the twenty-four thou- 

 sandth part of a second. Vibrations of such frequency 

 afford only a shrill squeak or chirp ; and Dr. Wollaston 

 has shown that there are many individuals with their 

 sense of hearing entire, who are altogether insensible to 

 such acute sounds, though others are painfully affected by 

 them. Nothing, as Sir John Herschel remarks, can be 

 more surprising than to see two persons, neither of them 

 deaf, the one complaining of the penetrating shrillness of 

 a sound, while the other maintains there is no sound at 

 all. Dr. Wollaston has also shown that this is true also 

 of very grave sounds, so that the hearing or not hearing 

 of musical notes at both extremities of the scale seems to 

 depend wholly on the pitch or frequency of vibration con- 

 stituting the note, and not upon the intensity or loudness 

 of the noise. This affection of the ear sometimes appears 

 in cases of common deafness, where a shrill tone of voice, 

 such as that of women and children, is often better heard 

 than the loud and deeper tone of men. 



Dr. Wollaston remarked, that when the mouth and 

 nose are shut, the tympanum or drum of the ear may be 

 so exhausted by a forcible attempt to take breath by the 

 expansion of the chest, the pressure of the external air 

 upon the membrane gives it such a tension, that the ear 

 becomes insensible to grave tones, without losing in any 

 degree the perception of sharper sounds. Dr. Wollaston 

 found, that after he had got into the habit of making the 

 experiment, so as to be able to produce a great degree of 



* Art. SOUND, Encyd. Metrop. 110. 



