THE EYE 



351 



Varieties of Lenses. Lenses are of three kinds: 



1. Spherical.") 



2. Cylindrical. 



3. Prismatic. - 



The spherical and cylindrical are either convex or concave, while 

 the prismatic may be plain or may be ground concave or convex. 

 Any two or three of them may be combined together into one lens. 



Oculists and opticians usually indicate the convex lens by a plus 

 sign and the concave lens by a minus sign. The focus of the lens 

 is measured by a unit known as a diopter." A lens of one diopter 

 has a principal focus one meter in length. (About 39 inches.) A 



FIG. 231. The range of perfect vision through a flat lens on the left and a 

 meniscus lens on the right. 



lens of two dioptrics would mean a focus of half a meter, and so 

 on. Thus a lens marked +3D signifies a convex lens of 3 dioptrics, 

 and a lens marked 3D signifies a concave lens of 3 dioptrics. 



In far-sighted eyes the rays of light tend to meet behind the 

 retina, but by the use of a convex lens placed before the eyes they 

 can be made to meet on the retina. (Fig. 230.) 



Near sightedness is caused by the rays of light meeting at a 

 focus in front of the retina. Concave lenses placed before the eyes 

 cause the rays of light to spread and again meet further back on 

 the retina. The lens must be more or less concave according to the 

 individual need. (Fig. 226, 227.) 



Flat and Meniscus Lens. There are two types of lenses used 

 for the eye. One is called the flat lens and the other is the meniscus. 



