VARIATIONS OF SENSIBILITY. 529 



sensibility, indeed, only by slow degrees. By 

 remaining in the dark its sensibility is still 

 farther increased, and a faint light will ex- 

 cite 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 painful sensation. 



This law of vision was usefully applied by Sir 

 William Herschel in training his eye to the 

 acquisition of extraordinary sensibility, for the 

 purpose of observing very faint celestial objects. 

 It often happened to him, when, in a fine winter's 

 nioht, 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 surround- 

 ing objects, by means of a black hood, the sen- 

 sibility of the retina was so much increased, that 

 when a star of the third magnitude approached 

 the field of view, he found it necessary imme- 

 diately to withdraw his eye, in order to preserve 

 its powers. He relates that on one occasion the 

 appearance of Sirius announced itself in the 

 field of the telescope like the dawn of the morn- 

 ing, increasing by degrees in brightness, till the 

 star at last presented itself with all the splendour 

 of the rising sun, obliging him quickly to re- 

 treat from the beautiful but overpowering spec- 

 tacle. 



VOL. II. M M 



