. Spectacles and eyeglasses, their forms, mounting, and proper adjustment . eometrical centers of the spectacles. Inother words, the geometrical center of the spectacle eyeand the optical center of the spectacle lens coincide, andthe center of the pupil for each eye lies directly behindthem. Regarding decentering, some confusion is apt toarise because the word is used in two different connections. 36 THE PRINCIPLES OF SPECTACLE FITTING. 37 If the visual axis pass to the temporal side of the opticalcenter of a glass held before an eye, then, with respect tothat eye, the glass is said to be dece
. Spectacles and eyeglasses, their forms, mounting, and proper adjustment . eometrical centers of the spectacles. Inother words, the geometrical center of the spectacle eyeand the optical center of the spectacle lens coincide, andthe center of the pupil for each eye lies directly behindthem. Regarding decentering, some confusion is apt toarise because the word is used in two different connections. 36 THE PRINCIPLES OF SPECTACLE FITTING. 37 If the visual axis pass to the temporal side of the opticalcenter of a glass held before an eye, then, with respect tothat eye, the glass is said to be decentered in. If thevisual axis pass to the nasal side of the optical center of theglass, the latter is decentered out. Similarly a glass maybe decentered in any other direction. When speaking ofspectacles, however, without reference to the eyes of thewearer, they are said to be decentered in when theiroptical centers lie to the inner side of their geometricalcenters; decentered out when the optical centers areto the external side of the geometrical centers, etc. A Fig. Spectacles with Lens Decentered G show the position of the geometrical centers; O O, that of the optical centers. glance at Fig. 17, which represents a pair of spectaclesdecentered in, will make clear what is meant. From the above it will readily be seen that when it isdesired that a patient wear decentered lenses, the effectmay be obtained in either of the two ways; first, by decen-tering the lenses in their frame; second, by displacingthem, together with their frames, from what I have de-scribed as the normal position. The first method hasthe disadvantage of increasing the weight of the glass,while the second limits the field of binocular vision. Inpractice, the second method should be employed to thegreatest extent possible without unduly interfering withbinocular vision for the distance at which the spectacleswill be used, and, should still farther decentering be re-quired, the method first mentioned
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