SELF-ADJUSTMENTS OF THE EYE. 



27. In the case of eyes whose convergent power is so great as- 

 to bring pencils proceeding from distant objects to a focus short of 

 the retina, and which therefore, for such distant objects, require 

 the intervention of divergent lenses, distinct vision will be attained 

 without the interposition of any lens, provided the object be placed 

 at such a distance that the divergence of the pencils proceeding 

 from it shall be such that the convergent power of the eye bring 

 them to a focus on the retina. 



Hence it is that eyes of this sort are called short- sighted, 

 because they can see distinctly such objects only as are placed at 

 the distance which gives the pencils proceeding from them such a 

 divergence, that the convergent power of the eye would bring 

 them to a focus on the retina. 



28. If it be desired to ascertain the focal length of the divergent 

 lens which such an eye would require to see distant objects- 

 distinctly, it is only necessary to ascertain at what distance it is 

 enabled to see distinctly the same class of objects without the aid 

 of a lens. A lens having a focal length equal to this distance will 

 enable the eye to see distant objects distinctly, because such a lens 

 would give the parallel rays a divergence equal to the divergence 

 of pencils proceeding from a distance equal to its focal length. 



29. Persons are said to be more or less near-sighted, according 

 to the distance at which they are enabled to see objects with 

 perfect distinctness, and they accordingly require, to enable them 

 to see distant objects distinctly, diverging lenses of greater or less 

 focal length. 



As persons who are enabled to see distant objects distinctly 

 have the power of accommodating the eye so as to see objects at 

 ten or twelve inches' distance, so short-sighted persons have a 

 similar power of accommodation, but within proportionally smaller 

 limits. Thus a short-sighted person will be enabled to see 

 distinctly objects placed at distances from the eye varying from 

 four or five inches upwards, according to the degree of short- 

 sightedness with which he is affected. 



30. The two opposite defects of vision which have been mentioned , 

 arising from too great or too little convergent power in the eye, 

 may arise, either from a defect in the quality of the humours or in 

 the form of the eye. Thus near-sightedness may arise from too 

 great convexity in the cornea or in the crystalline, or it may arise 

 from too great a difference of density between the aqueous humour 

 and the crystalline, or between the crystalline humour and the 

 vitreous, or both of them ; or, in fine, it may arise from defects 

 both of the form and of the relative densities of the humours. 



31. In a certain class of maladies incidental to the sight, the- 

 humours of the eye lose in a greater or less degree their trans- 



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