828 THE EYE AND VISION [CH. LVIII. 



the retractor lentis. In cephalopods the same occurs, but the retractor lentis is 

 absent ; here the approach of the lens to the retina is brought about by an alteration 

 of intra-ocular tension. In Amphibia and most snakes, the eye at rest is focussed 

 for distant objects; in accommodating for near objects the lens, by alteration of 

 intra-ocular tension, is brought forward, that is, the distance between it and the 

 retina is increased. There appear to be not a few animals in all classes which do not 

 possess the power of accommodation at all. Indeed, Barrett states this is so for 

 most mammals. 



DEFECTS IN THE OPTICAL APPARATUS 



Under this head we may consider the defects known as (1) 

 Myopia, (2) Hypermetropia, (3) Astigmatism, (4) Spherical Aber- 

 ration, (5) Chromatic Aberration, and (6) Presbyopia. 



The normal (emmetropic) eye is so adjusted that at rest parallel 

 rays are brought exactly to a focus on the retina (1, fig. 529). 

 Hence all objects except near ones (practically ail objects more than 

 twenty feet off) are seen without any effort of accommodation ; in 

 other words, the far-point of the normal eye is at an infinite distance. 

 In viewing near objects we are conscious of the effort (the contraction 

 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. 529). 



1. Myopia (short-sight), (4, fig. 529). This defect is due to an 

 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. Eays 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. 529). 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, 



