262 THE HUMAN BODY 



is attached a little in front of the edge. To see a nearer object the 

 ciliary muscle is contracted, and according to the degree of its 

 contraction slackens the suspensory ligament, and then the elastic 

 lens, relieved from the lateral drag, bulges out a little in the center. 

 When the eye is focussed for seeing a near object the circular 

 muscle of the iris contracts, narrowing the pupil, but this has 

 nothing directly to do with the accommodation. 



Short Sight and Long Sight. In the 

 normal eye parallel rays meet on the 

 retina when the ciliary muscle is com- 

 pletely relaxed (A, Fig. 89). Such eyes 

 are emmetropic. In other eyes the eye- 

 ball is too long from before back; in the 

 resting state parallel rays meet in front 

 of the retina (B). Persons with such 

 eyes, therefore, cannot see distant ob- 

 jects distinctly without the aid of diverg- 

 FIG. 89.-Diagram mustrat- in S (concave) spectacles; they are short- 

 ing the path of parallel rays s iqht e d or myopic. Or the eyeball may 



after entering an emmetropic 



(A), a myopic (#), and a hy- be too short from before back; then, in 



the resting state, parallel rays are 



brought to a focus behind the retina (C). To see even infinitely 

 distant objects, such persons must therefore use their accommodat- 

 ing apparatus to increase the converging power' of the lens; and 

 when objects are near they cannot, with the greatest effort, bring 

 the divergent rays proceeding from them to a focus soon enough. 

 To get distinct retinal images of near objects they therefore need 

 converging (convex) spectacles. Such eyes are called hypermetropic 

 or in common language long-sighted. 



Optical Defects of the Eye. The eye, though it answers ad- 

 mirably as a physiological instrument, is by no means perfect 

 optically; not nearly so good, for example, as a good microscope 

 objective. The main defects in it are due to: 



1. Chromatic Aberration. As already pointed out, the rays at 

 the violet end of the solar spectrum are more refrangible than those 

 at the red end. Hence they are brought to a focus sooner. The 

 light emanating from a point on a white object does not, therefore, 

 all meet in one point on the retina; but the violet rays come to a 

 focus first, then the indigo, and so on to the red, farthest back of 



