ZO THE OPTICAL DEFECTS OF THE EYE. 



Presbyopia, then, is not an optical defect of the nature of myopia 

 or hypermetropia, but is simply a lessening of the aceommodatiTe 

 power of the eye. 



It is supposed to depend upon, or to be caused by, the crystaline 

 lens becoming hardened as age advances, so that it does not yield 

 sufficiently to the contraction of the ciliary muscle. 



In a case of pure presbyopia where, for instance, the " near " point 

 is 12 inches from the eye, vision will remain normal for all points 

 beyond that distance. When the " near " point is 12 inches distant, 

 and the "far" point at an infinite distance, the accommodation is 

 only -j-^j-. Taking eight inches as the normal " near " point, |- would 

 represent the normal accommodation. Deducting -jL fi^em -^^ gives 

 the degree of presbyopia thus ■ ^ — r^ = -^t- The degree of 

 presbyopia in this case would then be ^^V. This fraction ^L also re- 

 presents the strength of the glasses necessary to correct the presby- 

 opia, namely 24 inch convex. Practically, we would probably 

 find that a pair of 30 inch convex would answer better, as the 

 weakest glass that can be worn with comfort, is the one that should 

 be prescribed. Again, if a person's "near" point be at 16 inches, his 

 presbyopia (i— tV=tV) ^i^l be -iV> ^'^^ a 16 inch convex lens would 

 enable him to read at 8 inches. 



" There can be no question as to the advisability and necessity of 

 affording far-sighted persons the use of spectacles. They should be 

 furnished with them as soon as they are in the slightest degree 

 annoyed or inconvenienced by the presbyopia. Some medical men 

 think that presbyopic patients should do without spectacles as long 

 as possible, for fear the eye should, even at an early period, get so 

 used to them as soon to find them indispensable. This is, however, 

 an error, for if such persons are permitted to work without glasses, 

 we observe that the presbyopia soon rapidly increases."* 



If, however, we call all cases presbyopia, where the " near" point 

 recedes to a greater distance than eight inches from the eye, it will 

 follow that we may have presbyopia in cases of myopia and hyper- 

 metropia. If a person's far point be at 20 inches from the eye he 

 would be called near-sighted and if his near point recedes to 10 

 inches from the eye, he would be ?i\so far-sighted. 



In some persons, as age advances, the " far " point also recedes so 



• J. Soelbcrg Wella. 



