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SUBJECTIVE DETERMINATION OF REFRACTIVE ERRORS

JACKSON CROSS CYLINDER

The Jackson cross cylinder is composed of equal-power, perpendicular plus and minus cylinders. The lens is mounted on a handle 45 to the axes, so that when it is rotated, the plus and minus cylinders exchange position. Placed before an emmetropic eye, the cross cylinder blurs the image. Placed before an ametropic eye, the cross cylinder does not alter the spherical equivalent, but it will enlarge or contract the interval of Sturm, blurring or clarifying the image, as it increases or decreases the net astigmatic ametropia.

Technique

The cross cylinder is used for subjective refinement of axis and power of cylinder after placing the best available estimate of refraction before the eye (retinoscopy, astigmatic dial test, or previous refraction). A line of letters two Snellen lines above the smallest seen with the patient's best acuity is displayed during the test. The cylinder axis is refined by rotating the cylinder, as …