46 DR KNOX on the Comparative Anatomy of the Eye. 



instance, which occurred in the person of a friend, whose powers 

 of vision were uncommonly strong, but whose eye, which was 

 dark, was so ill adapted to effect a sudden alteration of the fo- 

 cus, that, on hunting parties, he was always the last to discover 

 the game. It is extremely probable, that a principal part of the 

 phenomena depends on the rapidity of motion in the iris. I 

 think that I have noticed it oftenest in those in whom the iris 

 was of a blue colour. 



The same individual may, with some- attention, distinguish 

 the same object at different distances, the limits of which may 

 be assigned, with respect to each individual. The eye, there- 

 fore, must have the power of changing the position of its parts, 

 by some means or other, and these must be placed, either within 

 it, or exteriorly to it. So far as I am aware, no satisfactory theo- 

 ry has been offered concerning this accommodating power of the 

 eye, nor has any one fixed on the parts by which the internal 

 changes are effected. It is true, that in viewing near objects, 

 the pupil admits only the rays which are nearest to the axis, and 

 which are consequently the least diverging. This has been sup- 

 posed by some to be capable of accounting for the phenomena ; 

 but we shall, in the sequel, endeavour to shew, that this mode of 

 accounting for our power of viewing very near objects, though it 

 be essentially requisite to an accurate and complete theory of 

 vision, is yet totally inadequate to account for the whole of the 

 phenomena. This was the opinion of the late JOHN HUNTER, 

 very few of whose assertions have ever been confuted. 



It has been suggested by the celebrated Baron CUVIER, that 

 perhaps the limits of distinct vision are much more confined 

 than we imagine them to be ; and, it is probable, that, in many 

 cases, it only appears distinct, because it is assisted by the recol- 

 lection which we have of the object. Now, this should be con- 

 stantly borne in mind, for it prevents us from entertaining ex- 

 aggerated notions of the power of vision possessed by a variety 

 of animals. 



