COMMON THINGS SPECTACLES. 



a weak-sighted person, the candle c a visible object, and the 

 screen s's'the retina, upon which an indistinct image of the object 



G' 



(V 



S' 



5' 



- 



s depicted, and s s the place behind the retina, at which the 

 picture would be distinct. 



Now if another convex lens, G'G', which may represent 

 a spectacle glass, be placed before G G, it will have the effect of 

 bringing forward the place of the distinct image, and it will bring 

 that place more or less forward according as G' G' is more or less 

 convex. It is easy to conceive that its convexity may be such 

 that the image of the candle will be brought exactly to the position 

 of the screen s' s'. 



Thus it appears, that if the screen be misplaced, with relation 

 to the distinct picture of the candle, so as to be before it, a glass 

 G' G' of suitable convexity, placed before G G, will bring the dis- 

 tinct picture forward to the position of the screen, upon which it 

 will then be seen. 



This is a simple experiment which any one can try with a 

 candle, a sheet of paper, and two spectacle glasses. 



It perfectly represents the case of a weak-sighted person, and 

 the benefit they derive from convex spectacle glasses. 



If, however, the lens G' G' be too convex, the picture will be 

 brought too much forward, and it will be formed not on the screoii 

 s' s', but before it, and will consequently be indistinct. If, on the 

 contrary, the lens G' G' be not sufficiently convex, the picture will 

 not be brought so forward as the screen s' s', and will still be 

 indistinct upon it. 



Thus, between the relative positions of the distinct picture of 

 the candle, that of the screen s' s', and the convexity of the lens 

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