VARIATIONS OF SENSIBILITY. 371 



the sound of his fingers coming in contact with his head was 

 a fresh source of pain, producing an efTcct similar to that of 

 a pistol suddenly fired close to him. For a long time after, 

 when spoken to, even in the lowest whisper, he complained 

 of the distressing loudness of the sounds; and it was several 

 weeks before this excessive sensibility of the auditory nerves 

 wore off: by degrees, however, tlicy accommodated them- 

 selves to their proper function, and became adapted to the 

 ordinary impressions of sound. Some time afterwards, this 

 gentleman had a similar operation performed on the other 

 ear, and with precisely the same results; the same degree of 

 excessive sensibility to sounds was manifested on the resto- 

 ration of hearing in this ear as had occurred in the first; and 

 an equal time elapsed before it was brought into its natural 

 state. 



The most striking illustrations of the extent of this law 

 are furnished by the sense of vision. On entering a dark 

 chamber, after having been for some time exposed to the 

 glare of a bright sunshine, we feel as if we were blind; for 

 the retina, having been exhausted by the action of a strong 

 light, is insensible to the weaker impressions which it then 

 receives. It might be supposed that the contraction of the 

 pupil, which takes place on exposure to a strong light, and, 

 of course, greatly reduces the quantity admitted to the re- 

 tina, is a cause adequate to account for this phenomenon: 

 hut careful observation will show that the pupil very rapid- 

 ly enlarges to its full expansion when not acted upon by 

 liffht; while the insensibilitv of the retina continues for a 

 much longer time. It regains its usual sensibility, indeed, 

 only by slow degrees. ]5y remaining in the dark its sensi- 

 bility is still farther increased, and a faint light will excite 

 impressions equal to those produced in Ihc ordinary state of 

 the eye by a much stronger light; and while it is in this 

 state, the sudden exposure to the light of day produces a 

 dazzling and painful sensation. 



This law of vision was usefully applied by vSir William 

 Herschel in training his eye to the acquisition of extraordi- 



