838 MEASURE OF THE POWER OF ACCOMMODATION. 



optical effect produced by the change in the form of the lens that is 

 brought about by the muscular activity. 



These relations may first be considered in the emmetropic eye. In 

 the condition of rest, those (dotted) rays that pass in a parallel direction 

 from an infinite distance on the retina are united (Fig. 289, f). In 

 order to focus rays that come from the near point at a distance of 5 

 inches (p), the muscle of accommodation must exercise its full strength, 

 so that the lens may be made sufficiently convex. The power of ac- 

 commodation, therefore, produces an optical effect by increasing the 

 convexity of the previously passive flat lens (A) to an amount equal to 

 B; or in other words, it is as though a new convex lens B, had been 

 added to the original lens A. What, therefore, must be the focal dis- 

 tance of the lens B, in order that rays coming from the near point (5 

 inches) may be focused on the retina? Manifestly the lens B must 

 render the divergent rays parallel ; then A can focus them at f . Convex 

 lenses render parallel rays that come from their principal focus. In 

 the instance cited the lens consequently must have a focus of 5 inches. 

 Therefore, the normal eye, with a far point of infinity, and a near point of 



5 inches, has a power of 

 accommodation equiva- 

 lent to a lens of 5 inches 

 focus. If, now, the lens 

 is made more refractive 

 by its power of accom- 

 modation, the increase 

 may be readily elimi- 

 nated by placing before 



FIG 2gg the eye a concave lens 



that has an optical effect 

 exactly the opposite of 



that due to the increase of accommodation (B). Hence, it is possible 

 to use a lens of definite focus as the measure of the power of accom- 

 modation of the eye, that is for the optical effect produced by the 

 latter. According to Bonders the measure of the power of accom- 

 modation of an eye is the reciprocal value of the focal distance of a 

 concave lens that, when placed before the accommodated eye, so 

 refracts a bundle of rays coming from the near point (p) that it 

 appears to come from the far point (resting point of the eye). 



In accordance with the foregoing considerations, the power of accommodation, 

 then, is calculated by the following formula: - that is, the power 



of accommodation (expressed by the dioptric value of a lens of x inches focus) is 



equal to the difference between the reciprocals of the distances of the near (p) 



1 far points (r) from the eye. Examples: In the emmetropic eye, as already 



mentioned, p = 5; r = oo. Its power of accommodation is therefore = ] 



x ^ 



therefore x = 5; that is, it is equal to a lens of 5 inches focus. In a myopic eye, 



p = 4, r = 12; so that .= % fa and x = 6. Another myopic eye with 



p = 4, and r =20, has x = 5, in other words, normal accommodative power. 



Iwo eyes with different ranges of accommodation may have the same power of 



:ommodation. Example: One eye may have p = 4, r = oo; the other p =3, 



r - 4. For each eye = , or the power of accommodation of each is equal 



