CH. LV.] ERRORS OP REFRACTION. 763 



The account of accommodation as given in the preceding pages is true for 

 man and other mammals, birds and certain reptiles. 



Beer has, however, shown that in many animals lower in the scale, the 

 mechanism of accommodation varies a good deal, and is often very different 

 from that just described, consisting, in fact, in a power of altering the 

 distance between the lens and the retina. 



In bony fishes, the eye at rest is accommodated for near objects ; in 

 focussing for distant objects the lens is drawn nearer to the retina by a 

 special muscle called the retractor lentix. In cephalopods the same occurs, 

 but the retractor lent is 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, B.irrett states this is so for most mammals. 



DEFECTS IN THE OPTICAL APPARATUS. 



Under this head we may consider the defects known as 

 (i) Myopia, (2) Hypermetropia, (3) Astigmatism, (4) Spherical 

 Aberration, (5) Chromatic Aberration. 



The normal (emmetropic) eye is so adjusted that parallel rays 

 are brought exactly to a focus on the retina without any effort of 

 accommodation (i, fig. 585). Hence all objects except near ones 

 (practically all 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. 585). 



1. Myopia (short-sight) (4, fig. 585). This defect is due to an 

 abnormal elongation of the eyeball. T/he 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. Rays from 

 a point near the eye are exactly focussed on the retina, lint 

 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. 585). 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 



