34O ANATOMY AND PHYSIOLOGY 



convex lens of glass to provide for the lack of convexity. (See 

 Figs. 219 and 220.) 



In the condition known as astigmatism, the surfaces are ir- 

 regularly curved and they form a distorted image; the attempt to 

 correct this requires a constant effort which is very injurious to 

 the eye. The remedy is a lens of glass with counter-balancing 

 irregularities. 



Emmetropia is the condition of the normal eye. 



Myopia is near-sightedness. 



Hyperopia is far-sightedness. (This is congenital.) 



Astigmatism may be described as crooked-sigh tedness. 



Presbyopia is the far-sightedness of age (an acquired condition), 

 the tissues of the eyeball having lost their flexibility and resilience. 



The perception of color, or color vision, has not been quite 



FIG. 219. HYPERMETROPIA. PAR- FIG. 220. CORRECTION OF HYPER- 



ALLEL RAYS FOCUSED BEHIND THE METROPIA BY A CONVEX LENS. 



RETINA. (Brubaker.) (Brubaker.) 



clearly explained; consequently, we cannot state definitely the 

 cause of defective color vision, or color blindness. This is not 

 blindness to all colors, but usually to red or green. 



It is supposed that certain different chemic substances in the retina are 

 peculiarly sensitive to ether vibrations of different degrees of rapidity, 

 whereby impulses of a corresponding nature awaken in the brain the sensa- 

 tion of the various colors. In color blindness it is not difficult to imagine 

 that some of these sensitive chemic substances may be absent, thereby 

 making it impossible for the eye to perceive the corresponding color stimulus. 



Range of accommodation. By this is meant the distance 

 from the nearest to the farthest point at which an object can be 

 seen clearly. One can experiment for one's self with any small 

 object, as for example, with a pencil. By holding it very near 

 to the eye and gradually moving it away, the point will be found 

 where the image of the pencil is clear; this is the near point of 

 accommodation (punctum proximum oj vision). Moving it still 



