1 86 OPHTHALMOLOGY FOR VETERINARIANS 



ward, it again becomes upright. (As an example, place 

 a lantern-slide, inverted, in a ''magic lantern," and the 

 picture is projected upright on the screen.) 



Lenses used for the correction of refractive errors- 

 spectacle lenses — are the spheric, concave and convex, 

 and cylindric lenses. The spheric lenses are cut from 

 a sphere, that is, the surfaces have an equal radii of 

 curvature. Such lenses are called biconvex or bicon- 

 cave. Those with a plane surface on one side and a 

 curved surface on the other are called planoconvex 

 and planoconcave spherics. Cylindric lenses are cut 

 from a cylinder, and refract at right angles to the axis 

 of the cylinder. These are also convex and concave. 



Convex lenses are called plus (+) and concave lenses 

 are called minus ( — ). 



Lenses are now numbered according to their focal 

 length in metric measurements. A lens whose focal 

 length is i meter is called a i diopter lens. A lens of 

 2 meters focal length, 0.50 diopter; one of | meter focal 

 length, 2 diopters. A meter equals in the English system 

 39.37 inches. 



A plus one diopter spheric lens is designated thus, 

 4- i.oo D. S. A minus one diopter spheric lens is written, 

 -1. 00 D. S. In writing for plus or minus cylinders, 

 the same signs are used before the number, but in place 

 of the S. a C. is used, and, following this, the axis of 

 the cylinder is indicated, thus: +1.50 D. C. Ax. 90°; 

 -2.00 D. C. Ax. 180°. 



