SCIENCE OF COMMON THINGS. 



Explanation of far-sightedness. 



What sort of glasses do near-sighted persons wear ? 



If the cornea and crystalline lens be too convex (or 

 projecting), the person must wear double concave glasses 

 to counteract it. 



17*13 What is meant by " double concave glasses ?" 



Glasses hollowed-in on both sides. 



1 714 Why are old people far-sighted ? 



< Because the humors of their eves are dried up by age; 

 in consequence of which the cornea sinks in, or becomes 

 flattened. 



17*15 Why does the flattening of the cornea prevent persons seeing 

 objects which are near ? 



Because the cornea is too flat, and the image of ob- 

 jects is not completely formed when their rays reach the 

 retina / in consequence of which the image is imper- 

 fect and confused. 



Fig. 80 represents the man- 

 ner in which the image is 

 formed in the eye, when the 

 cornea or crystalline lens is 

 flattened. The perfect image 

 would be produced at m m, 

 behind the retina, and, of 



course, beyond the point ne- FJ ^ 



cessary to secure perfect vision. 



17*16 What sort of spectacle-glasses are suitable for old people f 



Double-convex glasses, or those which curve outwards 

 on both sides. These shorten the focus of the eye, and 

 produce an image upon the right point, the retina. 



1717* Why do near-sighted persons bring objects close to the eye in order 

 to see them ? 



Because the distance between the front and back of 

 the eye is so great, that the image of distant objects is 

 formed m front of the retina; but when objects are 

 brought near to the eye, their image is thrown farther 

 made to fall on the retina. 



1718 Why do old people hold objects far off in order to see them better t 



Because the distance between the front and back of 

 their eyes is not great enough / when, however, objects 



