10 FACULTIES OF BIRDS. 



expected the spot would become greatly larger ; but, 

 on the contrary, it remained the same. In another 

 experiment, he placed two candles, corresponding to 

 the extent of nerve of the eye, and then made the 

 highest change of its focus, expecting that, in conse- 

 quence, the outer candle would appear to move away 

 from him ; but in this also he was disappointed *. 



On the assumption of a change in the eye, several 

 suppositions have been made concerning the nature 

 of that change, most, if not all, of which are liable to 

 objections not easily repelled. We shall briefly advert 

 to the chief of these opinions. They refer to a change 

 in the globe of the eye, in the cornea, in the iris, 

 in the ciliary ring, and in the crystalline. Ac- 

 cording to the first of these, the globe of the eye is 

 compressed or relaxed by the surrounding muscles of 

 the eyeball, in order to render the axis of the globe 

 longer or shorter. But were this so the retina would 

 be puckered up into folds ; and, besides, we should 

 be more conscious of the change, inasmuch as the 

 muscles of the eye are voluntary f. 



It was the opinion of Monro, that the change partly 

 arose from the varied pressure of the eyelids upon the 

 ball, and he made several experiments to prove this. 

 He kept his eyelids wide asunder, and attempted to 

 read a book while he held it so near that the letters 

 were indistinct. He could not read it, in these 

 circumstances ; but, without moving his head or the 

 book, and bringing his eyelids within a fourth of an 

 inch from each other, he found he could read dis- 

 tinctly J. Sir Charles Bell, however, on keeping the 

 eyelids open, and using flat camel-hair pencils, as a 

 substitute for the eyelashes, found the same effect ; 



*Phil. Trans. 1793180] ; Medical Liter, p. 98-9. 



fHosack, Phil. Trans. 1794, p. 196; Knox, Edinb. Trans, x, 50. 



$ Three Treatises on the Eye. 



