■ THE OPTICAL DEFECTS OF THE EYE. 17 



ourselves to equi-concave lenses. An equi-concave lens is bounded by- 

 two surfaces, which are portions of the concave side of two circles 

 ■which have equal radii. 



Fig. 9. A, B, one of the concave surfaces of the lens. C is the 

 centre of curvature, and C, R the radius of curvature. When parallel 

 raySfc P, P, strike one surface of the lens, they have a divergence upon 

 leaving the second surface of the lens, as if they proceeded from the 

 centre of curvature, C, which, in an equi-concave lens, is also the 

 principal focus of the lens. C, R, is the focal length of the lens. 

 In a convex lens, the focus is measured behind the lens ; in a concave 

 lens, it is measured in front of it. If we call the focus of the convex 

 lens positive, we must call the focus of the concave lens negative. 

 When parallel rays of light fall upon a convex lens, they are converged 

 to a focus. When they fall upon a concave lens, they are made to 

 diverge. A convex lens enlarges, and a concave diminishes the appa- 

 rent size of objects. The focal length of a convex lens is measured 

 behind ; and that of a concave lens, in front of the lens. They are, 

 therefore, entirely opposite in all their properties ; and, for this reason, 

 a convex lens is called a positive lens ; and a concave one, a negative 

 lens. Or, shorter still, they are indicated by the plus (4-) and minus 

 (— ), algebraic symbols; thus, + 5, and — 5 ; or, -\- \, and — i. 

 To ascertain the focal length of a concave lens, we ascertain what 

 convex lens it will neutralize. 



1. In a myopic eye, parallel rays, as well as those that have a cer- 

 tain degree of divergence, are focussed in front of the retina; and, the 

 inverted image of distant objects being formed in the same position, 

 the picture upon the retina will be ill-defined, and vision for distant 

 object consequently indistinct. 



Patients with myopia complain that, although their vision for near 

 objects is perfect, they cannot see objects at a distance with any dis- 



VOL. XI. B 



