EFFECTS OF SHORT AND LONG SIGHT. 



such, spectacles the object can consequently be seen distinctly 

 without being brought near to the eyes. If, when the spectacles 

 are interposed, the object be brought as near the eyes as would be 

 necessary for distinct vision without spectacles, the vision will be 

 indistinct ; because, in that case, the effect of the glasses will be 

 to throw the distinct picture behind the retina, which, without 

 the glasses, would have been upon it. 



When persons are not very short-sighted, they generally read 

 or work without spectacles, but require their aid when they walk 

 abroad or move in society in large rooms, because the book or the 

 objects of their work can, without inconvenience, be placed at the 

 moderate distance from their eyes which is sufficient to throw the 

 focus back upon the retina, but the more distant objects at which 

 they look when walking abroad or in large rooms are beyond the 

 proper limit of distance, and the focus, being before the retina, 

 must be thrown back by concave spectacles. 



14. When an object is placed near the eyes of a weak-sighted 

 person, the focus is behind the retina, and the picture on the retina 

 is consequently indistinct. If the object be gradually removed 

 to a greater and greater distance, the focus, according to what 

 has been explained, will approach the retina nearer and nearer, 

 and, if the sight be not too weak, it will come upon the retina 

 when the object is removed to a certain distance from the eye. 

 In this case, however, owing to the greater distance of the object, 

 stronger illumination is required, and it is found, accordingly, 

 that when weak-sighted persons hold a book at arms-length from 

 the eyes, they are obliged, at the same time, to place a strong 

 light near the page. 



Eyes, which are not of very weak sight, have sufficient power to 

 bring the picture of all objects, whose distance exceeds three or 

 four feet, upon the retina. But for nearer objects, the picture, 

 being behind the retina, requires to be brought forward by the 

 interposition of convex spectacles. The nearer the object looked 

 at is, the more convex ought the glasses to be, and hence it comes 

 that very weak-sighted persons require to be provided with more 

 than one pair of spectacles, those adapted to more distant objects 

 being less convex, and those adapted to nearer objects more so. 



When the weakness of sight is so limited that the pictures of 

 distant objects fall upon the retina, those only of nearer ones 

 being behind it, the eye is said to be FAB-SIGHTED, in contra- 

 distinction to the opposite defect, by which distinct vision is only 

 obtained by the closer proximity of the object. 



15. Spectacles consist of two glass lenses mounted in a frame so 

 as to be conveniently supported before the eyes, and to remedy 

 the defects of vision of naturally imperfect eyes. 



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