. The clinical use of prisms; and the decentering of lenses . ct is unvarying,and dependent entirely on the deviating angle ofthe prism, Fig. 31. (2,) With binocular vision theprism may be too strong to be overcome, in whichcase there is diplopia with an inconstant distancebetween the images, which approach each otherspasmodically with every effort to overcome theprism. (3,) The diplopia may be overcome andthe object appear single, though its position inspace is misjudged through an angle practicallyequivalent to half the deviating angle of the prism. 5 66 Chapter IX. ADJUSTMENT OF PRISMS IN T
. The clinical use of prisms; and the decentering of lenses . ct is unvarying,and dependent entirely on the deviating angle ofthe prism, Fig. 31. (2,) With binocular vision theprism may be too strong to be overcome, in whichcase there is diplopia with an inconstant distancebetween the images, which approach each otherspasmodically with every effort to overcome theprism. (3,) The diplopia may be overcome andthe object appear single, though its position inspace is misjudged through an angle practicallyequivalent to half the deviating angle of the prism. 5 66 Chapter IX. ADJUSTMENT OF PRISMS IN THETRIAL-FRAME. If the apex and base of a circular prism arecorrectly marked, there is no difficulty in setting itwith the base-apex line either exactly vertical orhorizontal. But if there should be any doubt abouttheir precise position, the following simple ex-pedient will at once enable us to detect and correctany malposition. If the base-apex line is wishedto be horizontal, as with the right hand prism ofFig. 36, hold the trial-frame a few inches from the. Fig. 36.—Two prisms in a trial-frame. eyes, so that, as represented in the figure, the upperextremities m m m m, of its sockets coincide to allappearance with a horizontal line a b c, upon thewall, door, or window : if this line appears con- ADJUSTMENT IN TRIAL-FRAME. 67 tinuous, the prism is correctly set, but if that part ofit visible through the prism appears disjointed fromthe rest, as d in Fig. 36, the apex is either too highor too low, according as the apparent disjointment isupwards or downwards. In the figure the apex istoo high, and the correction is therefore easily madeby rotating the prism so as to depress the apex tillthe line appears continuous. If the base-apex lineis wished to be vertical, as with the left hand prismof the same figure, the horizontal line on the wallmust be supplemented by another at right angles toit, as efiVi the figure : again adjusting the frame asbefore to the horizontal line of sight
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