. Spectacles and eyeglasses, their forms, mounting, and proper adjustment . THE PRINCIPLES OF SPECTACLE FITTING. 47 To fulfil this requirement of perpendicularity to thevisual axis, the lenses of spectacles used only for distanceshould lie in a vertical plane; that is, they should facedirectly forward, as shown in Fig. 23. Since the visualaxes are directed downward and forward when near work isdone below the level of the eyes, glasses for near mustface downward and forward, as in Fig. 24, in order thatthe plane in which they lie shall be perpendicular to thoseaxes. Furthermore, in viewing near
. Spectacles and eyeglasses, their forms, mounting, and proper adjustment . THE PRINCIPLES OF SPECTACLE FITTING. 47 To fulfil this requirement of perpendicularity to thevisual axis, the lenses of spectacles used only for distanceshould lie in a vertical plane; that is, they should facedirectly forward, as shown in Fig. 23. Since the visualaxes are directed downward and forward when near work isdone below the level of the eyes, glasses for near mustface downward and forward, as in Fig. 24, in order thatthe plane in which they lie shall be perpendicular to thoseaxes. Furthermore, in viewing near objects the visual axesare directed inward and toward each other. This will re-quire the glasses to face inward also, as represented in , so that they come to lie in different planes, instead of inthe same plane, as formerly. Fig. When constant glasses are prescribed, the lensesshould be placed midway between the proper facing fornear and that for distance glasses. Then, though the lensis not exactly properly inclined either for distant vision ornear work, the result of such slight obliquity to the visualaxis is unimportant, since, as a reference to Table V willshow, it is only in the higher degrees of obliquity that theincrease in power, and especially the development of cylin-drical effect from spherical lenses, is rapid. Moreover, byslightly bending the neck a moderate degree of obliquityof the glasses to the visual axis may be removed withoutdiscomfort to the wearer. The position of bifocal glasses should also be betweenthat proper for near and for distance glasses, but nearer 48 SPECTACLES AND EYEGLASSES. that of the stronger glass. This will generally be the nearglass, as convex bifocals are much more frequently pre-scribed than concaves, and such glasses should face only alittle less downward than glasse
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