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r that the layman does not know of its presence and thus has no ground for consulting the doctor。 For example； it is well known that a large amount of ear wax in the aural passage may cause all sorts of ringing and sighing in the ear； and may even produce real hallucinations。 Yet a person having an abnormal amount of ear wax may be otherwise absolutely sound。 How is the need of a physician to be guessed in such a case？ Again； the perforation of the drum； especially when it follows a catarrh； may cause a definite auditory illusion with regard to the sound of voices； or the illusion may be effected by the irritation of the skin in the ear passage； or by anemia； or by a strong carotid pulse and a distention of the bloodvessels； as happens in alcoholism。 Many people become abnormally sensitive to sound at the beginning of fevers。 Women at the time of their climacterium hear all kinds of voices。 Inasmuch as this soon stops； the abnormality and incorrectness of their audition is hard to establish。 Childbirth； too； makes a difference。 Old； otherwise conscientious midwives claim to have heard unborn children breathe and cry。
Examples of this sort of thing are innumerable and they teach that whenever any questionable assertion is made about a thing heard the doctor must be called in to determine whether the witness heard it under abnormal； though not diseased conditions。 Again； merely accidental or habitual general excitability tends to intensify all sounds； and whether the witness under consideration was in such condition can be determined only by the expert physician。
The illusions of hearing which completely normal people are subject to are the most difficult of all。 Their number and frequency is variously estimated。 The physician has nothing to do with them。 The physicist； the acoustician and physiologist do not care about the criminalist's needs in this matter； and we ourselves rarely have time and opportunity to deal with it。 As a result our information is very small； and no one can say how much is still undiscovered。 One of my friends has called my attention to the fact that when the beats of the clock are counted during sleepiness； one too many is regularly counted。 I tested this observation and my experience confirmed it。 If； now； we consider how frequently the determination of time makes the whole difference in a criminal case and how easily it is possible to mistake a whole hour； we can get some notion of the importance of this illusion。 Its explanation is difficult and it may be merely a single instance of a whole series of unknown auditory illusions resting on the same basis。 Another and similar phenomenon is the ‘‘double beat of the hammer。'' If you have an assistant strike the table with a hammer while you hold both ears with your fingers and then open them half a second or a second after the blow； you hear the blow again。 And if you open and shut your ears quickly you can hear the blow several times。 This is explained through the fact that a number of reflections of the sound occur in the room； and that these are perceivable only by the unfatigued ear。 The explanation is unsatisfactory because the experiment is sometimes successful in the open。 Taken in itself； this matter seems very theoretical and without practical value。 But this kind of action may occur automatically。 It is well known that swallowing closes the Eustachian tubes for a moment； especially if done when lying down。 Now； if this occurs during a blow； a shot； etc。； the sound must be heard twice。 Again； it may easily happen that because of the noise a man wakes up half asleep and； frightened； swallows the collected saliva； then this accident； which in itself seems unimportant； may lead to very significant testimony。 Such occurrences are not infrequent。
The intensity of a sound already heard may be of considerable influence。 Certain experimenters have indicated the remarkable character of slightly intensive effects of sound。 If you hold a watch so far from the ear as to hear it clearly but weakly； the sound decreases until finally it is not heard at all； and after awhile it is again heard； etc。 This may lead to hearing distinct sounds made up of many tones； and need not evince any great illusion with regard to the ticking of a watch。 But the thing may occur also in connection with more powerful and more distant sounds； e。 g。； the murmur of a brook； the rush of a train； the pounding in a distant factory。 Noises far removed are influenced by reflections of sound； waves of air； etc。； and it is possible that all kinds of things may be heard in a completely monotonous noise。 This can be easily learned by listening to the soft murmur of a distant brook at night。 Given the disposition and supposing the existence of the brook unknown； it is easy to hear in its monotonous murmur； human voices； sighs； shrieks； etc。
Another remarkable observation shows that in the dark very distinct things are heard during the playing of delicate instruments； such as mouth…organs。 The humming approaches and withdraws； then it comes on various sides； and finally one has the feeling that the whole room is full of humming and winging insects。 And this may go on indefinitely。 There is a large collection of reasons for this reduplication of monotonous sounds。 Everybody knows the accord of the olian harp which consists of identical notes； and the melodies which seem to lie in the pounding of the train on the rails。 This can become especially clear when one is half asleep。 If ever thinking begins to be ousted by slumber； the rhythmic pound begins to dominate consciousness。 Then the rhythm gets its appropriate melody which becomes progressively more intense； and if one grows suddenly wide awake one wonders why the clearly…heard music is missing。 Similarly； it is often asserted that a row of travelling wild swans make pleasant chords； although each swan is able to utter only one cry。 Difference in distance and alterations in the air cause the chords。
The difficulties in distinguishing the intensity or weakness of a sound are of importance。 Fechner learned from the violinist Wasilewski that he observed that a male choir of four hundred voices did not sound essentially louder than one of two hundred。 At the same time one clock is not heard at a great distance； a hundred clocks are heard。 One locust can not be heard eating； when 1000 eat they are heard； hence each one must make a definite noise。＇1＇ Early authorities have already indicated how difficult it is to distinguish the number of bells ringing together。 Even musicians will often take two or three to be five or six。
Certain dispositions make some difference in this respect。 The operating physician hears the low groaning of the patient after the operation without having heard his loud cries during the operation。 During the operation the physician must not hear anything that is likely to disturb his work； but the low groan has simply borne in upon him。 The sleeping mother often is deaf to considerable noise； but wakes up immediately when her child draws a deeper breath than usual。 Millers and factory hands； travellers； etc。； do not hear the pounding of their various habitual environmental noises； but t