8 3 6 



REFRACTIVE POWER OF THE NORMAL EYE. 



FIG. 285 and FIG. 286. Refractive Condition of the Normal 

 Eye, at Rest and in Accommodation. 



shortest distance at which an object can still be seen distinctly The 

 distance between these two points is called the range of accommodation. 

 Three types of eyes are distinguished: 



The normal (emmetropic) eye is so constructed that, when it is at 



rest, parallel rays (Fig. 

 2 8 5, rr) from objects at an 

 infinite distance come to a 

 focus (r t ) on the retina. 

 The far point therefore 

 equals oo. On the strong- 

 est effort of accommoda- 

 tion for near vision, during 

 which the lens increases 

 its convexity (Fig. 286, a), 

 rays come to a focus upon 

 the retina (p x ) that are 

 emitted by a point of 

 light (p) 5 inches from the 

 eye, that is the near point 

 is 5 inches ( i inch equals 2 7 

 mm. ) . The range of accom- 

 modation is, therefore, oo. 



2. The short-sighted (myopic, hypometric, long) eye (Fig. 287) is 

 unable, when at rest, to focus parallel rays on the retina. Such rays 

 cross within the vitreous (at o), and then diverge and form a circle of 

 diffusion on the retina. 

 The objects must be 

 at a distance of from 

 60 to 120 inches (at /) 

 from the resting eye 

 in order that the rays 

 may be united on the 

 retina. The resting 

 short-sighted eye is, 

 therefore, capable of 

 bringing only diverg- 

 ent rays to a focus 

 upon the retina. The 

 far point, therefore, 

 lies abnormally near. 

 By the most power- 

 ful effort of accom- 

 modation, objects 

 may be distinctly 

 seen at distances of 

 from 4 to 2 inches, or 

 even less. The near 

 point also is abnor- 

 mally close; the range of accommodation is diminished. 



....- r-.~'f*^!f 



FIG. 287 and FIG. 288. Refractive Condition of the Short-sighted' and 

 the Far-sighted Eye. 



Myopia is usually dependent upon an elongation of the eyeball, which is 

 congenital and often inherited. The correction of this anomaly' of refraction is 

 effected by the use of a concave glass, which causes parallel rays from a great 



