CONVEX AND CONCAVE GLASSES. 



G' G', there is a certain relation, such that only one particular 

 degree of convexity will bring the distinct picture upon the 

 screen. 



In the same manner, it follows that between the relative 

 positions of the retina, of the distinct picture formed behind it, 

 and the convexity of the spectacle glasses, there is a certain 

 fixed relation, such that such glasses only as have a particular 

 convexity will bring the distinct picture on the retina, and pro- 

 duce clear and distinct vision. 



10. Let us suppose now that the screen is placed as at s's', 

 fig. 2, behind the position at which the distinct picture of the 

 candle is formed. In this case it is required to throw the 



G' 



distinct picture backwards, and as it was brought forwards by 

 the interposition of a convex glass, it will be thrown backwards 

 by the interposition of a concave glass. Such a lens, G'G', 

 having the proper degree of concavity, being therefore placed 

 before G G, the distinct image will be seen upon the screen s' s'. 



As in the former case, there is a certain relation between the 

 relative positions of the distinct picture of the candle, of the 

 screen s' s', and the concavity of the lens G' G', so that only one 

 particular degree of concavity will throw back the distinct picture 

 to the screen. 



As in the former case, this experiment illustrates the case of a 

 short-sighted eye, and the remedy aflixed by the interposition of 

 a concave glass. The lens G G represents the eye, s' s', the retina, 

 and s s the place before it where the distinct picture is formed 



197 



