ON THE USE AND CHOICE OF SPECTACLES. 
135 
vision is only limited by failure of light. Its near point, 
originally very near the eye, as at N, gradually recedes along 
the dotted portion to n', and the eye has always the range of 
view from N or n' to f. The short-sighted or myopic eye has 
^ N,, p; 
its far point near at hand, as at p', and its near point still 
nearer, as at n''. It has only the range of vision between n" 
and f', and if it should become presbyopic, this small range is 
still more diminished, by n" receding to 
Fig. 4. 
c 
In order to counteract optical defects, we make use of lenses, 
most commonly of what are called spherical lenses. These, 
when convex, possess the power of bringing rays of light to a 
focus at some given distance. Thus, in fig. 4 the convex lens c 
Fig 6. 
receives parallel rays of light A A , and unites them in a focus at 
B, which is called the focal point, or principal focus of c ; and 
the distance from c to b is called the principal focal length. If 
this distance be six inches, we call c a six-inch lens. If the 
lens be concave, as in fig. 5, its effect is precisely the reverse ; 
