. A treatise on diseases of the eye . s used as the symbol ofthe prism dioptre: Pr. 1-^, 2^, etc. When the prisms are made of glasswhose index is about , the number of prism dioptres in any prismis practically the same as the number of degrees in the refracting anglesystem, so that the units in these two systems differ chiefly in nomencla-ture. The centrad differs from the prism dioptre in that the former expressesthe deviation in hundreddis of the radius as measured on an arc, whilein the dioptric system the deviation is measm-ed on the tangent of thearc. The difference between the two is
. A treatise on diseases of the eye . s used as the symbol ofthe prism dioptre: Pr. 1-^, 2^, etc. When the prisms are made of glasswhose index is about , the number of prism dioptres in any prismis practically the same as the number of degrees in the refracting anglesystem, so that the units in these two systems differ chiefly in nomencla-ture. The centrad differs from the prism dioptre in that the former expressesthe deviation in hundreddis of the radius as measured on an arc, whilein the dioptric system the deviation is measm-ed on the tangent of thearc. The difference between the two is very slight in the prisms used inophthalmology. The metre angle denotes the angular deviation which the visual lineof each eye undergoes in turning from the primary position (straight for-ward) in order that the eyes may converge to a point in the median lineand 1 meter distant. As this angle varies with the interocular distance,it is unsuitable for numbering prisms, though it is very useful in themeasurement of convergence. Fig. 56. Refraction of Diverg-ent Rays at a Plane In Fig. od,SOS re])resents a section of a s})liencal j)en(il of liglil which eii(«M-s (hedense substance at die plane surface SS. That part of the light-wavewhich travels along OA meets the surface sooner than that which travelsalong OB. While the latter is travelling the distance BS, the ray to // in the dense medium. Jt is necessary in the explanation 92 GENERAL PRIXCIPLES OF OPTICS of refraction to assume tliat the veloeity of light is less in dense thanin rare media—an assumption which is corroborated hy , then, the rate of propagation is less in the dense than in the raremedium, AH less than liS, so that the wave front after refraction isrepresented l)y the curved line SIIS. If this line is the arc of a circleits centre does not lie at O, but at some point, /, farther from the sur-face than O. ^Mathematical demonstration proves that SHS is notstrictly circul
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Keywords: ., bookcentury1900, bookdecade1910, booksubjecteyediseases, bookyear
