212 HAND-BOOK OF PHYSIOLOGY. 



objects more than twenty feet off) are seen without any effort of accom- 

 modation: in other words, the far-point of the normal eye is at an infinite 

 distance. In viewing near objects we are conscious of an effort (the con- 

 traction of the ciliary muscle) by which the anterior surface of the lens is 

 rendered more convex, and rays which would otherwise be focussed behind 

 the retina are converged upon the retina (see dotted lines, 2, Fig. 380). 



1. Myopia (short-sight) (4, Fig. 380). This defect is due to an 

 abnormal elongation of the eyeball. The eye is usually larger than 

 normal, and is always longer than normal; the lens is also probably too 

 convex. The retina is too far from the lens, and consequently parallel 

 rays are focussed in front of the retina, and, crossing, form little cir- 

 cles on the retina; thus the images of distant objects are blurred and 

 indistinct. The eye is, as it were, permanently adjusted for a near-point. 

 Kays from a point near the eye are exactly focussed in the retina. But 

 those which issue from any object beyond a certain distance (far-point) 

 cannot be distinctly focussed. This defect is corrected by concave glasses, 

 which cause the rays entering the eye to diverge; hence they do not 

 come to a focus so soon. Such glasses of course are only needed to give 

 a clear vision of distant objects. For near objects, except in extreme 

 cases, they are not required. 



2. Hypermetropia (long-sight) (3, Fig. 380). This is the reverse 

 defect. The eye is too short and the lens too flat. Parallel rays are 

 focussed behind the retina: an effort of accommodation is required to 

 focus even parallel rays on the retina; and when they are divergent, as 

 in viewing a near object, the accommodation is insufficient to focus them. 

 Thus in well-marked cases distant objects require an effort of accommo- 

 dation and near ones a very powerful effort. Thus the ciliary muscle 

 is constantly acting. This defect is obviated by the use of convex glasses, 

 which render the pencils of light more convergent. Such glasses are of 

 course especially needed for near objects, as in reading, etc. They rest 

 the eye by relieving the ciliary muscle from excessive work. 



3. Astigmatism. This defect, which was first discovered by Airy, 

 is due to a greater curvature of the eye in one meridian than in others. 

 The eye may be even myopic in one plane and hypermetropic in others. 

 Thus vertical and horizontal lines crossing each other cannot both be 

 focussed at once; one set stand out clearly and the others are blurred and 

 indistinct. This defect, which is present in a slight degree in all eyes, 

 is generally seated in the cornea, but occasionally in the lens as well; it 

 may be corrected by the use of cylindrical glasses (i.e. curved only in 

 one direction). 



4. Spherical Aberration. The rays of a cone of light from an ob- 

 ject situated at the side of the field of vision do not meet all in the same 

 point, owing to their unequal refraction; for the refraction of the rays 

 which pass through the circumference of a lens is greater than that of 



