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Dr. Scott Alison^ 



[Feb. 18, 



upon the other a sound-collecting cup. Two of these, held together 

 by means of a ring of ivory or steel, will make an excellent differential 

 stethophone or phonoscope. 



The differential stethophone was designed 

 with the view of collecting different sounds 

 from two parts of the body at the same time, 

 and conveying them separately to each ear. 

 It formed in reality a stethoscope for each 

 ear, and it differed from all binaural or 

 double stethoscopes hitherto employed, which 

 collected sound through one aperture or 

 cup only. Though the old binaural ste- 

 thoscopes could not be made differential in- 

 struments, the differential stethophone miglit 

 be made a binaural stethoscope, simply by 

 placing the two sound-collecting cups at the 

 same part of the chest. When this is done, 

 the same auditory result is obtained as when 

 the simple binaural stethoscope is employed, 

 viz. a full sound and a distinct auditory sensa- 

 tion, fuller than when one ear only is em- 

 ployed. The advantages sought by means 

 of the differential stethophone were, 1st, to 

 give facility in comparing the ifttensity of 

 the fine breath sound of the lung at two dif- 

 ferent parts of the chest at the same time ; 

 and 2nd, to ascertain with exactitude the 

 relative commencement and termination of 

 two sounds generated at different parts of the 

 thorax ; which hitherto was impracticable : 

 for, as is obvious enough, it is not possible to 

 have one ear at the same moment at two dif- 

 ferent parts or to have the two ears in the 

 same plane, which however is now virtually 

 effected by the differential stethophone. The second object was fully 

 obtained ; but the first was not secured when the two ears were simul- 

 taneously employed, though by using the two ears in succession this 

 great advantage of comparing the intensity of one part with the in- 

 tensity of another was fully gained. It was found that the weak or 

 defective respiratory sound of one part produced no sensation in the 

 ear to which it was conveyed when the stronger sound of another part 

 was communicated to the other ear. 



This failure of the differential instrument, though disappointing at 

 the moment, has led to the ascertaining of an important acoustic prin- 

 ciple and to the practical application of it in medicine, viz. that a 

 major impression made on one ear will prevent all consciousness or 

 perception of a minor impression made at the same time on the other ear, 

 by the same sound ; and that an impression on one ear which produces 

 a distinct sensation may be made to produce no sensation whatever, by 



