SIGHT. 571 



and Prof. Mile of Warsaw, that it changes the curvature of the 

 cornea. 



Prof. Mile, by a great number of accurate and satisfactory 

 experiments, for which I refer the reader to the original memoir J , 

 has arrived at the following conclusions. Their truth is inde- 

 pendent of his hypothesis of the dependence of the curvature of 

 the cornea on the contraction of the iris, and the opinion of Sir 

 D. Brewster or Dr. Knox might be substituted for it with equal 

 propriety. 



The eye does not see with equal distinctness objects at all dis- 

 tances, but only when they are within a certain distance. This 

 does not depend on external causes, such as the diminution of the 

 optic angle, and the obscuration of the object by the intermediate 

 air ; for, to see clearly and to see distinctly are not identical. 

 The causes of distinct vision are internal, and situated in the eye 

 itself. They are two in number : one disposes the eye for the con- 

 tinuous distinct vision, and the other for the transient distinct 

 vision of objects at different distances ; but neither of them can 

 act except within certain limits. These limits are greater for 

 the presbyope, or shortsighted, than for the myope, or long* 

 sighted, person. These adaptations both depend on the action 

 of the iris, which can at the same time act in two ways to pro* 

 duce two effects : first, by the contraction of its aperture, and, 

 secondly, by the flexion of the cornea ; the alteration of the size 

 of the pupil only, however, is visible. The adaptation of the eye 

 for the continuous distinct vision of objects contained within 

 certain limits is owing to the diffraction of the rays of light near 

 the edge of the aperture of the iris, in consequence of which there 

 are formed, by a single external luminous point, seve*aP|bciiii^ead 

 of one successively ranged in a line of a certain length ; so that 

 the object may change its distance within certain limits, and yet 

 one of its foci shall always fall on the bottom of the eye. This 

 focal length is inversely as the magnitude of the pupil. The 

 borders of indistinct objects appear radiated, and to the pheno- 

 menon of confusion is added the motion and multiplication of the 



1 De la cause qui dispose I ceil pour voir distinct ement les objets places a dif- 

 ferentes distances : par Jean Mile (traduit du Polonais). Magendie, Journal de 

 Physiologic, t, vi. p. 166. 



