Text-book of ophthalmology . Fig. 358. 747. Prisms.—Prisms are employed for glasses either alone or com-bined with Lenses. The number engraved upon prisms indicates theirrefracting angle3; the deflection which the rays undergo amounts in thecase of weak prisms to one-half this angle. [For more precise statement seepage 766 and for uses of prisms § 672 passim and pages 757 and 794. P.] 1 (Prisms arc often and much better numbered by their deflecting power measured in centrada or En-,,, dioptric* \ ctntrad (denoted by the rijm V > ia the one-hundredth pari of the arc which is equali length to
Text-book of ophthalmology . Fig. 358. 747. Prisms.—Prisms are employed for glasses either alone or com-bined with Lenses. The number engraved upon prisms indicates theirrefracting angle3; the deflection which the rays undergo amounts in thecase of weak prisms to one-half this angle. [For more precise statement seepage 766 and for uses of prisms § 672 passim and pages 757 and 794. P.] 1 (Prisms arc often and much better numbered by their deflecting power measured in centrada or En-,,, dioptric* \ ctntrad (denoted by the rijm V > ia the one-hundredth pari of the arc which is equali length to the radiua (i » an angle whose tangenl ia 0 01, i. a , corresponds to a deflection of l cm. al l metre. The terms, pra< tii-aiiv interchangeable, since a prism of n eentrada and one of n pnsin-dioptnea are bomaltu(jst; precisely equivalent to a primn of n° refracting angle.— THE THEORY OF GLASSES 837 [A prism always disp
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