. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, a.—An understanding of what is known asthe angle gamma is important, that the observer mayunderstand and appreciate the real or apparent position ofthe eyes when looking at a near or distant point. Figure 76shows the line O A (optic axis) and the optic center, ornodal point (N), is situated on this line in the posterior partof the crystalline lens. The line V M is really a secondaryaxis to this dioptric system of the eye, and unites the object(V) with the fovea centrali
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, a.—An understanding of what is known asthe angle gamma is important, that the observer mayunderstand and appreciate the real or apparent position ofthe eyes when looking at a near or distant point. Figure 76shows the line O A (optic axis) and the optic center, ornodal point (N), is situated on this line in the posterior partof the crystalline lens. The line V M is really a secondaryaxis to this dioptric system of the eye, and unites the object(V) with the fovea centralis at M ; this line is known as the visual line. The angleformed by the visual linewith the optic axis at thenodal point i/un be con-sidered as the angle gam-ma. * If the fovea centralis atM was situated on the opticaxis at A, then the visualline and optic axis wouldcoincide, and there wouldnot be any angle gamma. In hyperopia and emme-tropic eyes the outer ex-tremity of the visual line »„ hes 15, 7, or, in some in- r in. 77- - / > stances, as much as 10degrees to the side of the optic axis (averaging. * This is not a perfectly correct statement, as the real angle gamma is theangle formed by the line of fixation V R with the optic axis, R being ihecenter of fixation. The angle V N O and the angle V R () being so nearlyequal, are, for all intents and , consideretl as the same. ANGLE ALPHA. :5 about 5 degrees), and is spoken of as positive, andgiven the plus sign. In some long myopic eyes, however,the outer extremity of the visual line may lie to the outerside of the optic axis, when it is spoken of as negative,and given the minus sign. To demonstrate the angle gamma, the patient is told tolook at the point of a pencil or pen held in the hand ofthe surgeon, at about 13 inches distant (A in Figs, jy and78). If the angle gamma is positive, the eyes will appeardivergent to the observer,who looks at the positionof the poles of the corneaor centers of the p
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Keywords: ., bookcentury1900, bookdecade1900, bookidref, booksubjecteyeglasses
