COMMON THINGS SPECTACLES. 



when no glass is used. When G'G' is interposed between tho 

 object and the eye, the distinct image is thrown back to s' s', the 

 place of the retina. 



11. The place at which the distinct image of a distant object is 

 formed by a lens, or by any other optical medium equivalent to a 

 lens, is called the FOCUS of the lens, and the distance of the focus 

 from the lens is called the FOCAL LENGTH of the lens. 



When the structure of the eye is perfect, therefore, its focus 

 must be on the retina, and its focal length will be the interior 

 diameter of the eye-ball. When the focal length of the eye is 

 greater than this the focus is behind the retina, and the eye 

 is weak-sighted or long-sighted. When the focal length is less, 

 the focus is before the retina, and the eye is short-sighted. 



12. If an object, a candle for example, placed before a convex 

 lens, be moved towards the lens, the place at which its distinct 

 picture is formed, that is its focus, will move from the lens, 

 so that the nearer the object is to the lens the further will its 

 picture be from it. It is easy to verify this by means of the 

 candle, the lens, and the screen. As the candle is moved nearer 

 and nearer to the lens, the place at which the screen will receive 

 a distinct picture of the flame will be farther and farther from the 

 lens, and in the same manner if the candle, being placed very 

 near the lens, be gradually removed farther and farther from it, 

 the place at which the screen will receive a distinct picture will 

 be nearer and nearer to the lens. 



This will explain some circumstances attending the vision of 

 near-sighted and weak-sighted persons, which are familiar to 

 every one. 



13. When a near-sighted person looks at a distant object, its 

 focus is within the eye-ball, before the retina, on which, conse- 

 quently, the picture is indistinct. But if the object be brought 

 gradually nearer and nearer to the eye, its distinct picture will 

 move more and more backward, according to what has been just 

 shown, and it will consequently approach nearer and nearer to the 

 retina, until at length the object is brought so near the eye, that 

 the distinct picture exactly falls upon the retina. The vision is 

 then perfect. 



It will thus be understood why near-sighted persons can see 

 objects distinctly, only when they are brought within a certain 

 distance of the eye. The more removed the focus of their eye is 

 from the posterior part, the nearer an object must be brought 

 before the picture is thrown back to the retina, and the person is 

 said to be so much the more near-sighted. 



Concave spectacle- glasses, in this case, have the same effect in 



throwing back the picture as the proximity of the object, and with 



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