786 THE EYE AND VISION [OIL LV1. 



abnormal elongation of the eyeball. The retina is too far from the 

 lens, and consequently parallel rays are focussed in front of the 

 retina, and, crossing, form little circles 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 on 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 (3, fig. 587). This is the reverse defect. The 

 eyeball is too short. 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 accommodation, and 

 near ones a very powerful effort, and the ciliary muscle is, therefore, 

 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 hyper- 

 metropic 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 

 object 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 those traversing its central portion. This 

 defect is known as spherical aberration, and in the camera, telescope, 

 microscope, and other optical instruments, it is remedied by the 

 interposition of a screen with a circular aperture in the path of the 

 rays of light, cutting off all the marginal rays, and only allowing the 

 passage of those near the centre. Such correction is effected in the 

 eye by the iris, which prevents the rays from passing through any 

 part of the refractive apparatus but its centre. The image of an 



