Page:Handbook of Ophthalmology (3rd edition).djvu/56

50 MYOPIA, TREATMENT. drawal of the near point, dependent upon advancing age. Pres- byopia develops in myopic as well as in emmetropic eyes. At the same time of life when the erametrope begins to need convex glasses the neutralizing concave glasses become too strong for the myope to use on near objects.

3. Generally the use of concave lenses is contra-indicated if from any cause the acuteness of vision is noticeably diminished. Corneal opacities or irregular astigmatism, cloudiness of the lens or vitreous, and all the causes of amblyopia, which oc«ur so frequently in high degrees of myopia, and are due to retinal or choroidal changes, contra-indicate the use of concave lenses. For distance, littlfe is usually gained by the correction of the myopia, and for near objects the hurtful influence of the concave lenses is to be feared, since the patients, in spite of them, approach closely to objects in order to obtain as large retinal images as possible, and then they must employ a tension of accommodation so much the stronger. To abstain as much as possible from work is the only advice to be given under these circumstances, as the causes of the amblyopia cannot be removed.

4. Finally, with reference to the relative range of accommoda- tion, the use of concave lenses ought not to be begun too late. Myopes not only acquire the habit of converging to the near point without accommodating, but also within the range of their distinct vision they associate with every degree of convergence of the visual axes only a relatively slight degree of accommoda- tion. Now, upon providing the eyes with concave lenses not only is the entire range of accommodation removed farther from the eye, but the position of the relative range of accommodation is also changed. Youthful eyes can generally accommodate them- selves to these changed conditions ; or where this is the only dif- ficulty, one can begin with weak lenses and proceed gradually to the neutralizing ones. At advanced age, however, myopes cannot, so easily as emmetropes, change the position of their rel- ative range of accommodation. The greater demand upon the available accommodation in near vision with concave glasses is generally very uncomfortable, and causes quick fatigue of accom- modation.

It is, moreover, probably because of the changed position in the relative range of accommodation that myopes who are accus-