Text-book of ophthalmology . I II III [Fig. 359.—Distortion Produced bt a Cylinder. Determination of Axis and Optical Center. (From Posey and Wright.)A right-angled cross, AB-CD, is seen through a glass containing a cylinder. If (I) the axis ofthe cylinder does not coincide with either AB or CD, the cross appears skewed, so that the arms nolonger make a right angle. The cross, however, is not displaced as a whole either to one side or theother (cf. Fig. 360). If now the glass is rotated until the axis of the cylinder coincides with one arm ABof the cross, the cross will appear right-angled and
Text-book of ophthalmology . I II III [Fig. 359.—Distortion Produced bt a Cylinder. Determination of Axis and Optical Center. (From Posey and Wright.)A right-angled cross, AB-CD, is seen through a glass containing a cylinder. If (I) the axis ofthe cylinder does not coincide with either AB or CD, the cross appears skewed, so that the arms nolonger make a right angle. The cross, however, is not displaced as a whole either to one side or theother (cf. Fig. 360). If now the glass is rotated until the axis of the cylinder coincides with one arm ABof the cross, the cross will appear right-angled and unbroken (II). The same thing will happen if theglass is rotated 90° further (III), so that the axis of the cylinder coincides with CD. In both II and IIIthe point on the glass where the two arms AB and CD, being undisplaced, appear to intersect is theoptical center.—D.] jA \A.
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