The numbers up to 100 make good tests if rightly selected and should be spoken dearly with the "reserve-air" which can be expelled from the lungs after an ordinary "tidal expira tion)); and they should be repeated by the per son examined in evidence that they have been correctly heard. The watch is a very variable, limited and imperfect test, which may be ill heard by ears otherwise perfect, well-heard by some with little useful hearing and imagined or falsely claimed to be audible when it is not, in a way not always easy of detection. All measurements of hearing for sounds or speech should be made with the eyes screened from seeing the approach of a sounding body or the lips" as many deaf persons do most successfully, yet at times uncon sciously. Medically, the best tests of hearing are by speech and by tuning-forks —the latter serving not only to measure the hearing or loss of it, hut to locate the seat of trouble as a prime requisite to successful effort for its re covery. A "continuous tone-series" of tuning forks or other instruments may be used by the expert to test the entire 10 or more octaves; and the aurist should always have several forks, although much may be learned with a single one. A fork of 200 to 500 double vibrations per second is the most useful and should be seven or more inches in length and weigh as many ounces: the latter is important as the loudness and duration of tone depend upon it. One of 30 to 5C and one of 2,000 double vibra tions should also be used, while intermediate tones have value. The lower forks should have thickened ends or clamps or pieces of rubber tubing on the tips to damp the "over-tones.' which may be heard by ears to which the funda mental note is inaudible. The fork should be struck by falling of its own weight through its own height upon a rather dead surface such as the top of the knee, and will thus give a tone of fairly constant strength; and testing a num ber of normal ears will show for each fork at what distance or for how many seconds it should be heard after a standard blow. Thus if a fork which a normal ear hears at 15 inches or for 100 seconds is heard by a deaf ear only 5 inches or for 30 seconds, 5:15 or 30:100 fairly represents the fraction of hearing remaining.* Ears with impairment of the percipient appa ratus of the internal-ear hear high sounds rela tively less as a rule; those with the more usual conduction-defect hear low tones worse. A case of "nerve-deafness" may hear no sound from the 2,000 fork— a case of tympanic catarrh none from the 50 fork; although all other tones are fairly or even perfectly heard.
A whistle devised by Galton giving tones from 4,000 up to 32,000 double vibrations is a practical and useful aid in studying the upper limits of hearing, as are the steel rods suspended by threads introduced by Koenig, since tuning-forks of such a pitch are unusual and costly. All of these tests should be made at the side of the head, but not so near that they may instead of heard. The other ear may need to be dosed with a down cushion or other ade quate cover and we must avoid several "deaf spots" nearly opposite the opening of the canal where the sound-waves by interference are weakened or lost.
Thus far we have considered tests of the hearing for sounds coming through the air in the usual way; but if the handle of a vibrating tuning-fork or other sounding instrument be rested upon the head it will be heard through the hone, and the sound will generally be much louder if the fingers are stuck into the ears.
If only one ear he stopped, it will be heard louder in this ear — so much so that the other generally fails to recognize its vibration, even though the fork is resting close behind or above it Normally this "bone-conduction') is only half as loud as "air-conduction"— the fork which should he heard for 100 seconds through the air in front of the ear is heard but 45 when resting behind it. It can usually be heard from the bridge of the nose or the teeth after it has become inaudible back of the ear; but when unheard here it should be audible for a further period if held close outside of the canal-open ing. Yet disease-conditions can affect the mat ter like stuffing the fingers in the ears; the obstructions in the conducting apparatus which bar out in greAter or less degree the outside air-vibrations shut in and echo back upon the percipient apparatus sounds produced or con ducted within the head. The deafer ear may hear the sound by bone-conduction better than its fellow. More than this — it can often hear better than a normal car, perceiving through the bone for 55 seconds, perhaps, the vibrations which ought to be heard for only 45. With this generally goes a marked loss of hearing through the air, which may be reduced from 100 seconds to 40, 20 or less. Under such con ditions bone-conduction will be better than air conduction and the sound will be louder as well as longer when resting on the head. This con stitutes Rinne's or Schwabach's test and is simplified by resting the handle of the lightly vibrating fork back of the ear and then plac ing it outside the opening with the question "louder hack or front. front or back?) The perception through the hone, especially at the bridge of the nose, ought to be nearly the same in duration as the feeling of the vibrations by the examiner's fingers; and Gardiner Brown saved time and confuSion by noting how many seconds before or after his fingers lost the vibration, it ceased to be heard in the car. Bone-conduction may greatly preponderate, in evidence of the conductive defect in an ear with marked loss of air-conduction, and yet this test or Schwabach's timing will show that the perceptive apparatus is also defective and loses the sound sooner than normal, instead of having it exaggerated. So we must test the actual, as well as the relative, perception through the hone.
Another delicate and important test may be here mentioned. The drum-cavity communi- c s with the throat back of the nose by a et-shaped tube called after its describer chins. This serves as a drain for the val of any mucus from the drum-cavity and as a ventilating passage for maintaining equal air-pressure on each side of the head. It is really a slit habitually closed, but opens in the act of swallowing or yawning, from the action of the muscles lifting the palate. If a sounding tuning-fork, then, is held before the nostrils as the patient swallows there should he a brief opening of these tubes ting freer passage of the sound to the ear. We thus determine the normal action or its lack in these very essential parts of the apparatus.