248 



THE HUMAN MECHANISM 



When the retina is so far away from the lens that, with 

 the muscle of accommodation completely relaxed and there- 

 fore the lens flattened to its utmost, light from distant 

 points comes to a focus in front of the retina, the retinal 

 image is blurred, and it is impossible for such an eye to see 

 distant objects clearly. To correct such vision it would be 



necessary to make the 

 lens still less convex, 

 and this the eye is un- 

 able to do. (Why?) 

 Such an eye is known 

 as myopic, or near- 

 sighted, and its defect 

 must be corrected by 

 the use of concave 

 glasses, which act as 

 if the lens were made 

 flatter, and so throw 

 the focus farther back 

 upon the retina. A 

 myopic eye generally 

 has clear sight for 

 very near objects be- 

 cause, as stated above, 



Course of the rays of light from a 

 distant point 



Through the emmetropic(^), the myopic (M), and 

 the hypermetropic (H) eye, the muscle of accom- 

 modation being relaxed. (The rays diverging 

 from a distant point would enter the eye practi- 

 cally parallel) 



FIG. 97. 



the nearer the object 

 the farther back is the 

 image formed. 



On the other hand, 

 the eyeball may be 

 too short, fore and aft (Fig. 97, ^T), so that, when the ciliary 

 muscle is relaxed, light from distant points has not yet been 

 brought to a focus when it reaches the retina (Jiypermetropia). 

 Such an eye must accommodate not only for near but also 

 for distant objects, and its muscle of accommodation can 

 never rest so long as the eye is being used. Moreover, to see 



