. Physiological optics : being an essay contributed to the American encyclopedia of ophthalmology . Taking the radius of curvature as 5 mms. and the index ofrefraction as %, it is necessary to calculate first of all the focal dis-tance, fM conjugate to the known posterior focal distance f2 = 19 expression is i + y3 = 4/3-i fx 19 5 1 1 4 3 1>1 fx 15 57 855 Hence fx = — 28816 mms. PHYSIOLOGICAL OPTICS The negative sign is of significance, for it indicates that the anteriorfocal point is on the same side of the refracting surface as the pos-terior focal point, indicating, therefore, a
. Physiological optics : being an essay contributed to the American encyclopedia of ophthalmology . Taking the radius of curvature as 5 mms. and the index ofrefraction as %, it is necessary to calculate first of all the focal dis-tance, fM conjugate to the known posterior focal distance f2 = 19 expression is i + y3 = 4/3-i fx 19 5 1 1 4 3 1>1 fx 15 57 855 Hence fx = — 28816 mms. PHYSIOLOGICAL OPTICS The negative sign is of significance, for it indicates that the anteriorfocal point is on the same side of the refracting surface as the pos-terior focal point, indicating, therefore, a virtual point back of theretina. This is in accord with the commonly known principle thatonly light converging toward the eye can be brought to a focus uponthe retina of an hyperopic eye not exerting its accommodation. Theseconditions are diagrammed in Fig. 4. FP and FA are conjugate foci;AFP = 19 mms. and FA, by calculation, is — 288 mms. This distance,288 mms., in dioptric equivalent is practically D; this eye has19 mms. instead of 20 mms. as its posterior focal length; we therefore. Fig. 4.—Illustrative of the Optical Conditions Existing in a Hyperopic Eye as to Conjugacy of Foci. conclude that 1 mm. decrease in the depth of the normal eye, repre-senting hypermetropia, indicates an axial error of D. The practitioner is herewith also furnished the basis for calcula-tions which may be of diagnostic value in conjunction with ophthal-moscopic examinations. We do not refer to the use of the ophthalmo-scope as an instrument for the measurement of refractive errors, butrather to its employment in determining differences in level betweenthe edge or ring of the optic disc and the center of the cup in, forexample, physiologic cup, glaucoma and choked disc. The differencein the lens quantity which the observer must turn up in his instru-ment in order to, let us say, clearly see in turn the edge and the centerof the disc, gives a measurement of fair accuracy upon the rela
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