The instrument is used without the stopper, when employed to ascertain the state of respiration; and with it, either to examine the action of the heart, or the signs afforded by the voice, in certain states of disease affecting the lungs, or the pleural membrane. In using the stethoscope, care is to be taken that it is kept perfectly flat upon the part to which it is applied, in order that sound may not escape, nor air be admitted; for which purpose it is convenient to hold it at its lower part, while the other end (with the even surface) should be in close contact with the ear, so that the aperture of the instrument be in direct communication with the internal ear. Silk covering, as causing a creaking sort of noise, and thick dress, as obscuring the sound of respiration, should be removed; but the linen or flannel dress may remain on the person without disadvantage. A beginner will be very apt to fail in his early trials, from inattention to some of these points, and from the difficulty which at first attends the tuition of the ear. A skilful use of the instrument requires much practice. It is not, as some may imagine, a simple matter of hearing a delicate sound. _Tact_ is necessary, but this will be acquired by perseverance. For two days of my own study, I was frequently at a loss to distinguish the peculiar sound or murmur of respiration, and almost despaired of success. I mention this circumstance, that others beginning with the practice of this instrument, may not be deterred from proceeding, by the awkwardness and disappointment which they at first encounter. At _La Charité_, the best informed students are great enthusiasts in the practice of this discovery, and regard it as an indispensable source of information in the diagnosis of the diseases of the chest. Such reliance on the stethoscope is very natural, when constant proofs are afforded of the accuracy of opinion formed by M. Laennec, and others, who are competent in using it. The proofs of sure diagnosis are derived from dissection; as, necessarily, the diseases of the lungs and of the heart, allow frequent opportunity of obtaining this test of the physicians’ judgment. I will relate one instance which came within my immediate knowledge; although equally strong proofs are so numerous, that the regular observer can no more doubt the merits of the stethoscope, than the mathematician can distrust a resolved problem. In the clinical record of a case of consumption, M. Laennec had stated his opinion, that the upper portion of one lung was the exclusive seat of ulceration. The patient died, and examination was made. The lungs were removed from the chest, and the expected evidence of disease did not appear. M. Laennec, making a closer inspection, discovered that a small part of the upper lobe had been left in the chest, and upon its removal, the ulceration was found exactly as he had predicted.