. A treatise on diseases of the eye . t of reversal to the exam-iners eye, which is 1 meter distant from the examined eye. If rotationof the mirror produces the appearance characteristic of the point ofreversal without any lens, the eye is myopic ID. If the shadow move-ment is with the mirror, and a convex lens of 1 D. neutralizes this move-ment, the eye is emmetropic (because a lens of 1 D. is required to produce1 D. of myopia). If 3 D. is required to neutralize the movement, theeye is hyperopic 2 I)., etc. But if a concave lens of 3 D. is required toneutralize a movement against the mirror-r
. A treatise on diseases of the eye . t of reversal to the exam-iners eye, which is 1 meter distant from the examined eye. If rotationof the mirror produces the appearance characteristic of the point ofreversal without any lens, the eye is myopic ID. If the shadow move-ment is with the mirror, and a convex lens of 1 D. neutralizes this move-ment, the eye is emmetropic (because a lens of 1 D. is required to produce1 D. of myopia). If 3 D. is required to neutralize the movement, theeye is hyperopic 2 I)., etc. But if a concave lens of 3 D. is required toneutralize a movement against the mirror-rotation, the eye is myopic4 1)., etc. In astigmia there is a separate point of reversal for each of the twoprincipal meridians. When the observer is at the point of reversalfor one principal meridian, there will be a movement of the shadow onrotating the mirror in the other principal meridian. The differencein the power of the lenses which place the observer at the points of reversalin the two meridians measures the astigmia. Fig. 79. Maddox double prism: a, front view; b, sectional view. Keratometry.—Keratometry or ophthalmometry, as it is usually called,consists in measuring the curvature of the anterior surface of the , it consists in estimating the corneal astigmia by measuringthe difference of curvature in the two principal meridians of the measurement is made by the method of double images. If weplace a Maddox double prism (Fig. 79) before one eye so that the apexof the prism bisects the pupil, and then view through this eye and theprism an object such as is represented in Fig. SO, we see two imagesof the object, and when the object is at a certain distance the two imageswill appear in contact (Fig. 81). In this case the prismatic displace-ment is exactly equal to the length (AB) of the object. Hence, if thedeflecting power of the prism and the distance are known, the lengthAB can be determined. Thus, if the prism has a deflecting power of1
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Keywords: ., bookcentury1900, bookdecade1910, booksubjecteyediseases, bookyear
