. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, c 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 pupils.(See Fig. JJ^) If the angle gamma isnegative, the eyes willappear convergent—tha
. Refraction and how to refract : including sections on optics, retinoscopy, the fitting of spectacles and eye-glasses, c 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 pupils.(See Fig. JJ^) If the angle gamma isnegative, the eyes willappear convergent—thatis, they appear to con-verge to a point in frontof the pencil. (See ) The amount of theangle gamma can bemeasured by using thearc of the perimeter heldhorizontally, the patientbeing placed in the sameposition as when having his field taken. To do this, while the eye fixes the centralpoint, the surgeon passes a candle-flame along the arc untilthe catoptric image of the flame is seen at the center of thepupil ; this position of the candle-flame on the arc is notedin degrees, which is the size of the angle Fig. 78. 86 REFRACTION AND HOW TO REFRACT. Angle Alpha.—This is the angle formed by the longaxis of the corneal ellipse with the visual axis. In theconsideration of this angle it must be remembered that thecornea resembles, in its central area, at least, an ellipsoidof revolution, with the shortest radius usually in the verti-cal meridian. The angle alpha is spoken of as positivewhen the outer extremity of the long axis of the cornea isto the outer side of the visual line, and negative when itis to the nasal side. CHAPTER Direct and Indirect Method. Ophthalmoscope.—From oipOaXpAx;, eye, and , to observe or view ; literally, to view an eye. Aninstrument used for studying the media and interior of theeye. The pupil of an eye in health appears to an observer asblack ; this is due to the fact that the observers eye doesnot ordinarily
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