THE EYE. 



as to bring the retina closer to the crystalline, when it is desired to 

 obtain a distinct view of distant objects. 



According to others, this change of form is only effected in the 

 cornea, which being rendered more or less convex by a muscular 

 action gives a greater or less convergent power to the aqueous 

 humour. 



According to others, the eye accommodates itself to different 

 distances by the action of the crystalline, which is moved by the- 

 ciliary processes either towards or from the cornea, thus transfer- 

 ring the focus of rays proceeding from it within a certain limit of 

 distance to and from the retina ; or, by a similar action of the 

 ciliary processes, the crystalline lens may be supposed to be 

 rendered more or less convex, and thus to increase or diminish 

 its convergent power. 



25. Whatever be the provisions made in the organisation of the 

 e J e > by which it is enabled to adapt itself to the reception of 

 divergent pencils proceeding from near objects, the power with 

 which it is thus endued has a certain limit. Thus eyes, which 

 see distinctly distant objects, and which therefore bring parallel 

 rays to a focus on the retina in their ordinary state, are not 

 capable of seeing distinctly objects brought nearer to them than 

 ten or twelve inches. The power of accommodating the vision to 

 different rays is therefore limited to a divergence not exceeding 

 that which is determined by the diameter of the pupil compared 

 with a distance of ten or twelve inches. Now, as the diameter of 

 the pupil is most contracted when the organ is directed to such 

 near objects, we may assume it at its smallest magnitude at one- 

 eighth of an inch, and therefore the divergence of a pencil pro- 

 ceeding from a distance of twelve inches would be ^th, and the 

 angle of divergence would therefore be very nearly half a degree. 



It may, therefore, be assumed that eyes adapted to the vision of 

 distant objects are in general incapable of seeing distinctly objects 

 from which pencils have greater divergence than this, or which is the 

 same, objects placed at less than ten or twelve inches from the eye. 



26. In the case of eyes whose convergent power is too feeble to 

 bring pencils proceeding from distant objects to a focus on the 

 retina, they will be in a still greater degree inadequate to bring 

 pencils to a focus which diverge from near objects; and con- 

 sequently such eyes will require to be aided, for near as well as 

 distant objects, by the interposition of convergent lenses. It 

 would, however, be necessary to provide lenses of different 

 convergent powers for distant and near objects, the latter requiring 

 a greater convergent power than the former ; and in general the 

 nearer the objects viewed, the greater the convergent power 

 required from the lens. 



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