HYPOTHESES ACCOUNTING FOR IT. 23 



individuals in whom the iris is wholly wanting have usually perfect vision 

 for near as well as distant objects. 



Volkmann and Hueck both agree in considering that in its quiescent 

 state the eye is adapted to the vision of objects situated at the furthest 

 point of distinct sight, and not, as has been generally supposed, of those 

 situated about midway between this and the point of distinct vision nearest 

 to the eye. In this case, therefore, in order to accommodate itself to the 

 vision of an object placed at any distance within the furthest point of 

 sight, the eye will require but one act, that, namely, of increasing its focal 

 distance in proportion to the nearness of the object under view : no act 

 will be requisite to adapt it to the perception of distant objects, for, in 

 reverting to its state of rest, it at once resumes its capacity for distant 

 vision, and retains it so long as its quiescent state continues. In proof of 

 this opinion Volkmann observes, that in the state of rest the axes of the 

 eyes are directed towards a point even considerably beyond the most dis- 

 tant point of distinct vision, and that, since changes in the position of the 

 axes usually correspond with changes in the adaptation of the eyes, it is 

 improbable that the meeting of the axes beyond the most distant point of 

 vision should coincide with an adaptation of the eyes for an object on this 

 side the point. According to Hueck this view will also explain the dis- 

 tinct formation of the image of distant objects on the retina after death ; 

 as also the far-sightedness induced by the action of hyoscyamus and of 

 belladonna. 



The tendency of most of the late observations on the subject of the 

 accommodating power of the eye is in favour of the view proposed by 

 Kepler, and countenanced by Professor Miiller,* that this power is 

 mainly due to some alteration either in position or form, or in both, 

 undergone by the crystalline lens. The arguments stated by Hueck in 

 favour of this view are, first, that if the eye is watched attentively from 

 the side, the iris will be observed to be bent forwards in the middle and 

 approximated closer to the cornea when a near object is viewed, and to 

 become flattened again when the sight is fixed upon a distant object. 

 And, secondly, that when the fresh eye of a dog is removed and placed 

 before a window, so that a distinct image of the window-frame through an 

 opening in the sclerotica, and an indistinct one of a smaller object, such 

 as a key, held nearer to the eye, are perceived, the latter may be rendered 

 distinct, and the former indistinct by drawing the lens forward with a 

 needle inserted through the margin of the cornea. With respect, how- 

 ever, to the mode in which this supposed approximation of the lens 

 towards the cornea during the vision of near objects is effected, different 

 explanations still continue to be offered. Burowf adopts that view, 

 according to which the forward movement of the lens is attributed to vas- 



* Physiology, p. 1150. t TourtuaTs Report (page xi), in MUller's Archiv. 1842. 



