SENSE OF HEARING TIME REQUIRED FOR PERCEPTION. 797 



the other. The interval between the two sensations is greater in some per- 

 sons than in others; for some can receive and be conscious of many impres- 

 sions, seemingly at the same moment ; whilst in others, a perceptible space 

 must elapse. The " personal equation " of each observer in an Observatory 

 has, therefore, to be determined and allowed for. 1 



653. Amongst other important offices of the sense of Hearing, is that of 

 supplying the sensations by which the Voice is regulated. It is well known 

 that those who are born entirely deaf, are also dumb ; that is, they do not 

 spontaneously or imitatively form articulate sounds, though not the least 

 defect may exist in their organs of voice. Hence it appears that the vocal 

 muscles are usually guided in their action by the sensations received through 

 the Ears, in the same manner as other muscles are guided by the sensations 

 received through themselves ; but when the former are deficient, the action 

 of the vocal muscles may be guided by the latter ( 537). That the per- 

 ceptions obtained by the auditory nerve should be capable of being more 

 quickly perceived and registered than those received through the optic is 

 not surprising, since the course of the former is much shorter than that of 

 the latter. The following table gives the results of several observers in 

 regard to the physiological time of the several senses of sight, hearing, and 

 touch ; or in other words the whole term occupied between the occurrence of 

 an event and its registration : 



For vision. For hearing. For common sensation. 



Hirsch, . . .0.2 0.149 0.182 hand. 



Hankel, . . . 0.2057 0.1505 0.1548 " 



Donders, . . .0188 0.18 0.154 neck. 



Wittich, 2 . . . 0.194 0.182 0.1301 forehead. 



By subtracting the physiological time for auditory impressions from those 

 for optical impressions, it appears that the rapidity of the conduction of 

 sensory impulses is comparatively slow. On the highest estimate not ex- 

 ceeding 7.35 metres per second, and on the lowest, amounting to only 1.068 

 metres per second. 



Tl<e Measure of Sensations. The observations made by Weber ($ 594) and others 

 on the several senses, have led to the establishment by Fechner of a psycho-physical 

 law 3 applicable to the quantitative sensations of temperature, pressure, muscular ex- 

 ertion, sound, light, and of fatigue. At first sight it would seem impossible to meas- 

 ure a sensation, but inasmuch as the excitation of any nerve which leads to sensation 

 is caused by external or physical conditions which we can vary or quantitatively 

 determine, we possess in this a means of measuring sensations. Thus, if a weight of 

 half an ounce be held in the hand, and another half ounce be added, a difference is 

 instantly felt ; but the addition of half an ounce to twenty pounds is not appreciated. 

 The same is true of light and sound. It would hence appear that for an excitation 



1 In all the best Observatories, an arrangement is now made for recording observa- 

 tions which supersedes the necessity of timing them by simultaneous attention to the 

 clock. The observer who is watching the transit of a star (for example) across the 

 meridian, simply presses a button at the moment when he witnesses its contact with 

 the cross wire. This pressure breaks an electric circuit, which is so connected with 

 a chronometer as to stop it instantaneously, nnd thus automatically to record the 

 precise time of the phenomenon. Since this plan has been adopted, it has been found 

 that the " personal equation " nearly or altogether disappears ; thus confirming the 

 view given above as to its dependence on the distraction of the attention between the 

 two objects of perception. For interesting papers on the subject of this paragraph, 

 see Mach, On the Sense of Time in the Ear, Moleschott's Unters., Bd. x, 1866, Heft 

 2. Also Mach, On the Accommodation of the Ear, in idem, p. 201. 



2 See Von Wittich's paper in Henle and Meissner'sZeitschrift, 1868, Bd. xxxi, p. 87. 



3 Fechner, Elemente der Psychophysik, 2 vols., Leipsic, 1860; Delboeuf, Re- 

 cherches Theoriques et Experimentales sur la Mesure des Sensations, Bruxelles, 1873; 

 Eibot, La Psychologic Physiologique en Allemagne, in La Rev. Scient., No. 24, 

 1874. This section is chiefly drawn from Wundt, Physiologic, 1873. 



