754 



EMMETROPIC AND MYOPIC EYES. 



When accommodating as much 

 a* possible for a near object, whereby the convexity of the lens is increased (fig. 

 ~ fmm a luminous Doint placed at a distance of 5 inches are still focussed 



on the retina, i.e., the near point 

 is = 5 inches (1 inch =27 mm.). 

 The range of accommodation, or 

 [" the range of distinct vision "], 

 therefore, is from 5 inches (10-12 

 cm.) toco . 



2. The short-sighted, myopic 

 eye (or long eye) cannot, when at 

 rest, bring parallel rays from in- 

 finity to a focus on the retina (fig. 

 539). These rays decussate within 

 the vitreous humour (at O), and 

 after crossing form diffusion circles 

 upon the retina. The object must 

 be removed from the passive eye 

 to a distance of 60 to 120 inches 

 (to /), in order that the rays may 

 be focussed on the retina. The 



The 



Condition of refraction in the normal passive eye and 

 during accommodation. 



passive myopic eye, therefore, can only focus divergent rays upon the retina. 

 /"<// point, therefore, lies abnormally near. With an intense effort at accommodation, 



objects at a distance of 4 to 



./.^-^:. : 



or even less, from 

 may be seen dis- 



2 inches 

 the eye 



tinctly. The near point, 

 therefore, lies abnormally 

 near ; the range of accom- 

 modation is diminished. 



Short-sightedness, or myopia, 

 usually depends upon congenital, 

 and frequently hereditary, elon- 

 pj <r -gg gation of the eyeball. This 



. * anomaly of the refractive media 



Myopic eye. j s eas jiy corrected by using a 



diverging lens (concave), which makes parallel rays divergent, so that they can then be brought 

 to a focus on the retina. It is remarkable that most children are myopic when they are born. 

 This myopia, however, depends upon a too-curved condition of the cornea and lens, and on the 

 lens being too near to the cornea. As the eye grows, this short-sightedness disappears. 



The cause of myopia in children is ascribed 

 to the continued activity of the ciliary muscle 

 in reading, writing, &c, or the continued 

 convergence of the eyeballs, whereby the 

 external pressure upon the eyeball is in- 

 creased. 



-:,-./ 3 - Tte long-sighted, hyperme- 

 tropic eye, hyperoptic (flat eye) when 

 at rest, can only cause convergent rays 

 to come to a focus on the retina (fig. 

 .540). Distinct images can only be 

 formed when the rays proceeding from 

 objects are rendered convergent by 

 means of a convex lens, as parallel rays 

 would come to a focus behind the retina (at/). All rays proceeding from natural 

 objects are either divergent, or at most nearly parallel, never convergent. Hence, 



Fig. 540. 

 Hypermetropic eye. 



