The Jackson cross cylinder. A reappraisal.

The Jackson cross cylinder is a frequently used but poorly understood tool for clinical refraction. A critical analysis of the principles of cross cylinder testing reveals several important features which should be understood for the proper use of this device. For example, a rarely discussed shortcoming of the cross cylinder is that it can fail to detect even large amounts of ocular astigmatism in certain situations. A little-known advantage of the cross cylinder, on the other hand, is its use in refining cylinder power at an incorrect axis. This particular property of the cross cylinder is useful when the correcting cylinder axis must be rotated toward 90 degrees, 180 degrees, or toward the old axis. The cross cylinder can be used to determine the optimal cylinder power for this new axis setting. Guidelines are also considered for the appropriate step sizes in axis rotation that should be used in cylinder axis refinement.