IX USK OF SPECTACLES 413 



sight can adjust their eyes so as to see distinctly objects 

 as near to the eye as five or six inches ; but the image of 

 an object brought nearer than this becomes bkuTed and 

 indistinct, because the " near limit " of adjustment is then 

 passed. They can also adjust their eyes for objects at a 

 very great distance, the indistinctness of the images of 

 objects very far off being due, not to want of proper focus- 

 sing, but to the details being lost through the minuteness 

 of the image. 



Some people, however, are born with, or at least come 

 to possess, eyes in which the "near limit" of adjust- 

 ment is much closer. Such persons can see distinctly ob- 

 jects as near to the cornea as even one or two inches ; 

 but they cannot adjust their eyes to objects at any great 

 distance off. Thus many of these " near-sighted " people, 

 as they are called, cannot see distinctly the features of a 

 person only a few feet off. Though their ciliary muscle 

 remains quite relaxed so that the suspensory ligament 

 keeps the lens as flat as possible, the arrangements of the 

 eye are such that the image of an object onlj^ a few feet 

 off is brought to a focus in front of the retina, somewhere in 

 the vitreous humour. By wearing concave glasses these 

 near-sighted people are able to bring tlie image of distant 

 objects on to the retina and thus to see them distinctly. 



The cause of near-sightedness is not always the same, 

 but in the majority of cases it appears to be due to the 

 bulb of the eye being unusually long from back to front. 

 K, in the water-camera described above, when the lens 

 and object were so adjusted that the image of the object 

 was distinctly focussed on the screen, the box were made 

 longer, so that the screen was moved backwards, the 

 distinctness of the image on it would be lost. 



Some people are bom really "long-sighted," inasmuch 

 as they can see distinctly only such objects as are quite 

 distant ; and indeed have to contract their ciliary muscles, 

 and so make their lens more convex even to see these. 

 Near objects they cannot see distinctly at all unless they 



