. The clinical use of prisms; and the decentering of lenses . , however, though more difficult to use,possessed the advantage of giving a dioptric mea-surement true for media of any index of refraction,while the prism measure only gives the phy-sical inclination of the surfaces. The latter instru-ment, shown in Fig. 62, is very simple in its a lens be placed within it, and be moved aboutin the horizontal plane till the index points to zero,the tooth in the centre of the foot of the indexpoints to the optical centre. If, on the other hand,the geometrical centre be placed just below thist
. The clinical use of prisms; and the decentering of lenses . , however, though more difficult to use,possessed the advantage of giving a dioptric mea-surement true for media of any index of refraction,while the prism measure only gives the phy-sical inclination of the surfaces. The latter instru-ment, shown in Fig. 62, is very simple in its a lens be placed within it, and be moved aboutin the horizontal plane till the index points to zero,the tooth in the centre of the foot of the indexpoints to the optical centre. If, on the other hand,the geometrical centre be placed just below thistooth, the index points to the number which mea-sures the physical angle of the virtual prism withwhich the lens is combined. This does not initself, however, give us much information about the ANALYSIS OF SPECTACLES. 149 effect of the spectacles on the visual axes, for thegeometrical centre may not be directly in front ofthe eye. If we measure from the centre of thebridge a distance equal to the distance of thecentre of the eye from the median plane, and .f. Fig. 62.—The prism measure, used to discover the opticalcentre of a lens. mark this distance by an ink dot on the glass,then we can place the said ink dot under the toothand read off what prism is virtually combined withthe lens at this point, directly in front of the this way we discover the prismatic effect of the ISO PRISMS. spectacles without any calculation. This appli-cation of the instrument, though not mentioned byits inventor, makes it as useful to the surgeon asit is to the optician, and renders any descriptionof the analyser almost unnecessary, except for afew who may have had it constructed, or who wishgreat exactness with pebble lenses, as well as withglass ones. The analyser, which is shown in Fig. 63, ha? aplate of glass, c d, which is movable from side to letters a b indicate a narrower plate of glass, also k, \f
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Keywords: ., bookcentury1800, bookdecade1890, bookpublisher, booksubjectlenses
