796 OF THE ORGANS OP THE SENSES, AND THEIR FUNCTIONS. 



pass, that the power of hearing very faint sounds and of recognizing their 

 source becomes augmented to a wonderful degree in those individuals who 

 are obliged to trust to the knowledge thus acquired for the direction of their 

 own actions ; whilst, in .like manner, the power of distinguishing slight dif- 

 ferences in the pitch of sounds, may be so cultivated (where it is not con- 

 genitally deficient) as to attain an intensity that seems very extraordinary 

 to those who have not accustomed themselves to listen for them. The gen- 

 eral cultivation of this sense is perhaps most remarkable in blind persons, 

 who have enabled themselves, by reliance upon it, to walk about freely, 

 even in the crowded thoroughfares of the metropolis ; and who are not only 

 able to judge of the habits of individuals whom they meet, by the sound of 

 their footsteps (at once recognizing, for instance, the footstep of a policeman 

 on duty), but can even tell when they are passing a stationary object (such 

 as a lamp-post), provided it be as high as the ear or nearly so, by the 

 reverberation of the sound of their own footsteps, and can discriminate be- 

 tween a lamp-post and a man standing still in the position of one, by the 

 same means. 1 The effect of habitual attention in increasing the discrimina- 

 tive power for impressions of one particular kind, is perhaps best seen in the 

 ability which is possessed by certain Conductors of orchestral performances, 

 to detect the slightest departure from time or tune in the sound of any one 

 of (perhaps) a hundred instruments that are simultaneously sounding, and 

 to fix without hesitation upon the faulty instrumentalist. Seebeck, indeed, 

 affirms that accomplished musicians can detect a difference in pitch be- 

 tween two sounds which only differ by y^Voth in the number of their vibra- 

 tions. 2 According to the researches of MM. Renz and Wolff, 3 our faculty 

 of estimating differences in the intensity of sounds is much less perfect. 

 When the intensities of two sounds are to one another as 7 to 10, the dif- 

 ference can be distinguished, but not when they are in the relation of 9 to 

 10. The advantage resulting from the simultaneous employment of the 

 two ears in distinguishing minute differences in the intensity and pitch of 

 sounds, is well shown in the experiment suggested by M. Weber, of holding 

 two watches in the hand before one ear, when it will be found that the suc- 

 cessive sounds can be distinctly heard, though they cannot be distinguished 

 one from the other, whilst if one be held in front of each ear the two sounds 

 can be clearly discriminated. 



652. Facts of much interest have been ascertained, some of which have 

 been already alluded to ( 471), in regard to an occasional difference in the 

 rapidity of the perception of sensory impressions received through the Eye 

 and through the Ear respectively. These facts are the result of comparisons 

 made amongst different Astronomical observers, who may be watching the 

 same visual phenomenon, and " timing " their observations by the same 

 clock ; for it has been remarked, that some persons see the same occurrence, 

 a third or even a half of a second curlier than others. There is no reason 

 to suppose from this, however, that there is any difference in the rate of 

 transmission of the sensory impressions in the two nerves. The fact seems 

 rather to be, that the Sensorium does not readily perceive two impressions 

 of different kinds with equal distinctness; and that, when several such im- 

 pressions are made on the senses at the same time, the mind takes cognizance 

 of one only, or perceives them in succession. When, therefore, both sight 

 and hearing are directed simultaneously to two objects, the communication 

 of the impression through one sense will necessarily precede that made by 



1 See the account of a blind hoot-lace seller given by Mr. H. Mayhew, in his 

 London Labor and the London Poor, vol. i, p. 402. 



2 See Ludwig, Physiologic, vol. i, p. 380; and Beclard, Physiol., 1862, p. 1858. 



3 Archiv f. Phys. Heilk., 1856. 



