. The clinical use of prisms; and the decentering of lenses . nclined to oneanother at a moderate angle. The strength ofa prism depends on the size of this angle, whichvaries therefore in different prisms, and also onthe refractive properties of the material from whichthe prism is made. Since the terminology of prisms in optics, andespecially in ophthalmology, is so different fromthat of prisms in geometry and physics, it will bewell to notice in some detail the terms employed. Refracting Surfaces.—By refraction ismeant the deflection, or sudden bending from itsformer course, of a ray of light
. The clinical use of prisms; and the decentering of lenses . nclined to oneanother at a moderate angle. The strength ofa prism depends on the size of this angle, whichvaries therefore in different prisms, and also onthe refractive properties of the material from whichthe prism is made. Since the terminology of prisms in optics, andespecially in ophthalmology, is so different fromthat of prisms in geometry and physics, it will bewell to notice in some detail the terms employed. Refracting Surfaces.—By refraction ismeant the deflection, or sudden bending from itsformer course, of a ray of light as it enters (otherthan perpendicularly) the surface of a rarer ordenser medium. The body of a prism does notrefract light, since rays travel through its sub-stance in straight lines. It is only as they enter orleave the prism, and thus pass from one medium to another that they undergo refraction. It is for I PRISMS. this reason that the two plane polished faces arecalled the refracting surfaces. In Fig. I theyare represented by the faces acfb and a Angle of Refraction.—We have seen that the twoplane faces of a prism are made to slope towardseach other so as to contain between them a mode-rate angle. This angle is called the angle ofrefraction, because the total effect of the prismon the direction of a ray of light depends on thesize of this angle, as well as on the refracting powerof the material from which the prism is recently, clinical prisms have had the numberof this angle in degrees scratched upon them, sothat prisms marked, , with i° or 2° were knownto have their refracting surfaces inclined to eachother by i° or 2° respectively. Trial cases usedgenerally to be provided with a series of prisms, THEIR GEOMETRICAL PROPERTIES. marked in this way, from i° or 23 up to 12° or angle of refraction is the one item the opticianneeds to know in order to grind a prism. He firstsets a pair of compasses so that their legs shallenclose the presc
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Keywords: ., bookcentury1800, bookdecade1890, bookpublisher, booksubjectlenses
