570 SIGHT. 



tender and impatient of light from the^ absence of this pig- 

 ment." * 



Adaptation of the eye. When the eye is directed to objects at 

 a distance, it is unable, at the same time, to see distinctly objects 

 which are near. Some change is therefore requisite to accommo- 

 date the eye to see perfectly at different distances. That the eye 

 thus adapts itself to distinct vision by a voluntary act, is evident 

 from the following simple experiment. If you hold a pencil near 

 the eye and direct the attention steadfastly to it, distant objects 

 will appear confused ; but if, without changing the position of the 

 eye, the attention be transferred to these distant objects, they will 

 become distinct and the pencil will appear confused. Many hy- 

 potheses have been proposed to explain the actions by which this 

 adaptation of the eye is effected, but none yet advanced has been 

 deemed entirely satisfactory. Kepler supposed that the eye 

 elongates itself in the direction of its axis in proportion as the ob- 

 ject to be viewed is nearer. Dr. Jurin, Sir E. Home, Ramsden, 

 and others, regarded the cornea as capable of changing its form 

 and curvature, becoming more convex when the eye is adjusted 

 to see nearer objects. Descartes, Pemberton, Albinus, Hunter, 

 Olbers, and Dr. Young have endeavoured to prove that the crys- 

 talline lens is capable of moving in the direction of the axis of the 

 eye so as to vary its distance from the retina, that it advances 

 to see near objects, and recedes to become fit for seeing distant ob- 

 jects. La Hire and Le Roy supposed that the mobility of the 

 pupil is alone sufficient to account for the phenomena. Dr. Ma- 

 gendie hastily denies that any adaptation is necessary, " because," 

 says he, " the picture seen at the back of the eye of an ox, when 

 the sclerotic is scraped away, is equally distinct at whatever dis- 

 tance the object is placed before it." 



On paying attention to the phenomena, it will be observed that 

 the effort of adapting the eye to a near object is invariably at- 

 tended by the contraction of the pupil. Several eminent writers 

 have hence considered the motion which causes the adaptation to 

 be a consequence of the mobility of the iris. Dr. Knox thinks 

 that the contraction of the iris alters the curvature of the crys- 

 talline lens ; Sir D. Brewster, that it displaces the crystalline ; 



k " I have spoken of Albinos at large in my work, De Generis Humani 

 Varietate Nativa, ed. 3. p. 274. j and in my dissertation, De Oculis Leucce- 

 thiopum" 



