Stethoscope

"As to your practice, if a gentleman walks into my rooms smelling of iodoform, with a black mark of nitrate of silver upon his right forefinger, and a bulge on the right side of his top-hat to show where he has secreted his stethoscope, I must be dull, indeed, [...]" 
(Aventure I, A SCANDAL IN BOHEMIA pg. 4)

The first version of the stethoscope was invented in France in 1816 by René Laennec. The device  consisted of a rigid wooden tube and was monaural. The flexible and binaural stethoscope as we know it, and probably the one referred to in the novel, was  invented only in 1852 by George Cammann, who perperfected the design of the instrument for commercial production. The device has become the standard for clinical practice ever since.