470 THE SENSORIAL FUNCTIONS. 



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 retina, is a cause adequate 

 to account for this phenomenon ; but careful obser- 

 vation will show that the pupil very rapidly enlarges 

 to its full expansion when not acted upon by light; 

 while the insensibility of the retina continues for a 

 much longer time. It regains its usual sensibility, 

 indeed, only by slow degrees. By remaining in 

 the dark its sensibility is still farther increased ; 

 and a faint light will excite impressions equal to 

 those produced in the 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 painfid sensation. 



This law of vision was usefully applied by Sir 

 William Herschel in training his eye to the acqui- 

 sition of extraordinary sensibility, for the purpose 

 of observing very faint celestial objects. It often 

 happened to him, when, in a fine winter's night, 

 and in the absence of the moon, he was occupied 

 during four, five, or six hours in taking sweeps of 

 the heavens with his telescope, that, by excluding 

 from the eye the light of surrounding objects, by 

 means of a black hood, the sensibility of the retina 

 was so much increased, that when a star of the 

 third magnitude approached the field of view, he 



