THE POWER OR FORCE OF ACCOMMODATION. 755 



a long-sighted person, when the eye is jxissive, i.e., is negatively accommodated, 

 cannot see distinctly without a convex lens. When the ciliary muscle contracts, 

 slightly convergent, parallel, and even slightly divergent rays may be focussed, 

 according to the increasing degree of the accommodation. The far point of the 

 eye is negative, the near point abnormally distant (over 8 to 80 inches), while the 

 range of accommodation is infinitely great. 



The cause of hypermetropia is abnormal shortness of the eye, which is generally due to 

 imperfect development in all directions. It is corrected by using a convex lens. 



[Defective Accommodation. In the presbyopic eye, or long-sighted eye of old 

 people, the near point is farther away than normal, but the far point is still 

 unaffected. In such cases, the person cannot see a near object distinctly, unless it 

 be held at a considerable distance from the eye. It is due to a defect in the 

 mechanism of accommodation, the lens becoming somewhat flatter, less elastic, and 

 denser with old age, while the ciliary muscle becomes weaker. In hypermetropia, 

 on the contrary, the mechanism of accommodation may be perfect, yet from the 

 shape of the eye the person cannot focus on his retina the rays of light from a near 

 object. In presbyopia the range of distinct vision is diminished. The defect is 

 remedied by weak convex glasses. The defect usually begins about forty-five 

 years of age.] 



Estimation of the Far Point Snellen's Types. In order to determine the far point of an 

 eye, gradually bring nearer to the eye objects which form a visual angle of 5 minutes (e.g., 

 Snellen's small type letters, or the medium type, 4 to 8, of Jaeger), until they can be seen dis- 

 tinctly. The distance from the eye indicates the far point. In order to determine the far point 

 of a myopic person, place at ,20 inches distant from the eye the same objects which give a visual 

 angle of 5 minutes, and ascertain the concave lens which will enable the person to see the objects 

 distinctly. To estimate the near point, bring small objects (e.g., the finest print) nearer and 

 nearer to the eye, until it finally becomes indistinct. The distance at which one can still see 

 distinctly indicates the far point. 



Optometer. The optometer may also be used to determine the near and far points. A small 

 object, e.g., a needle, is so arranged as to be movable along a scale, along which the eye to be 

 investigated can look as a person looks along the sight of a rifle. The needle is moved as near 

 as possible, and then removed as far as possible, in each case as long as it is seen distinctly. 

 The distance of the near and far point and the range of accommodation can be read off directly 

 upon the scale (Grafe). 



389- FORCE OF ACCOMMODATION. Force. The range of accommodation, which is 

 easily determined experimentally, does not by itself determine the proper power or force of 

 accommodation. The measure of the latter depends upon the mechanical work done by the 

 muscle of accommodation, or the ciliary muscle. Of course this cannot be directly determined 

 in the eye itself. Hence, this force is measured by the optical effect, which results in consequence 

 of the change in the shape of the lens, brought about by the energy of the contracting muscle. 



In the normal eye, during the passive condition, the rays coming from infinity, and there- 

 fore parallel (which are dotted in fig. 541), are focussed upon the retina at/. If rays coming 

 from a distance of 5 inches (p. 756) . 



are to be focussed, the whole avail- >^>llk 



able energy of the ciliary muscle must "l^^^^i^wt~~~----^ 



be brought into play to allow the lens ^ -""'""""' j^^^^fii --^_ 



to become more convex, so that the -> <^"~ || || \ J ^^-~--- 



rays may be brought to a focus at "---^^^ ^R^^Bf ^^^ 



*. The energy of accommodation, "^~^^~^----_^Pi,^^^' ^^-^^" 



therefore, produces an optical effect in - - xvV /"""" 



as far as it increases the convexity >^r 



of the anterior surface of the passive 



lens (A), by the amount indicated by p-^ g , j 



B. Practically, we may regard the * 



matter as if a new convex lens (B) were added to the existing convex lens (A). What, therefore, 



must be the focal distance of the lens (B), in order that rays coming from the near point (5 inches) 



may be focussed upon the retina at/ ? Evidently the lens B must make the diverging rays coming 



from j9, parallel, and then A can focus them at/. Convex lenses cause those rays proceeding from 



their focal points to pass out at the other side as parallel rays ( 385, I. ). Hence, in our case, 



the lens must have a focal distance of 5 inches. The normal eye, therefore, with the far point 



= 00, and the near point = 5 inches, has a power of accommodation equal to a lens of 5 inches 



