(Directions for use with a plus cylinder phoropter)

Place the Jackson cross cylinder device in the “axis” position; that is, with an imaginary line connecting the 2 small thumb-wheels aligned parallel to the axis of the cylinder battery (Figure I-25). That alignment is indicated by the arrows on the axis knob and by indicator lines or arrowheads near the aperture of the phoropter. The cross cylinder should click into place. Present a binary comparison between the 2 positions of the cross cylinder by rotating 1 of the thumb-wheels. Notice how this interchanges the indicator dots (red and white) on the cross cylinder lens. Continue the binary comparisons, and after each, turn the plus cylinder axis toward the orientation of the white dots on the cross cylinder lens in the position where the patient reports clearer vision. The endpoint is often vague (“about the same,” “equally bad,” and so on). Watch for increased hesitation by the patient, or a reversal of the progression of the axis as you make successive binary comparisons. The process is self-correcting—if you go too far in 1 direction, the patient’s responses will indicate that the cylinder axis should be brought back to a previous direction.

(Directions for use with a plus cylinder phoropter)

Turn the cross cylinder turret 45° to the “power” position, as indicated by an imaginary line connecting the “P” marks on the lens rim running parallel to the (newly determined) cylinder axis. Flip the cross cylinder to provide a binary comparison. Notice how either the red dots or the white dots on the cross cylinder will line up along the cylinder axis. If the patient’s preferred position of the cross cylinder places the white dots along the cylinder axis, increase the plus cylinder power. (If the preferred position of the cross cylinder places the line through the white dots perpendicular to the cylinder axis, so the line connecting the red dots lies parallel to the cylinder axis, reduce the plus cylinder power.) Repeat until you reach an endpoint, where the views through the alternate positions of the cross cylinder are “about the same.” [At this stage, the view may still not be very good!] When in doubt, go with the numerically weaker choice of plus cylinder power.

KEY STEP: For each 0.5 D (2-click) increase in plus cylinder power as determined by these cross cylinder flips, reduce the sphere power by 0.25 D (1 click). After you have located the endpoint, remove the cross cylinder from the optical pathway. If all is well, the patient should experience a considerable improvement in visual acuity.

As with sphere power, the determination of cylinder axis and cylinder power by means of binary comparisons of the view through the cross cylinder in its various positions is self-correcting. If you make an error in adjusting the lenses, the next comparison you offer the patient will create a contrast between views that is even clearer than the last.

If you are using loose trial lenses in a trial frame instead of a phoropter, you will have to align the hand-held Jackson cross cylinder (Figure I-27) with the tentative axis of the correcting cylinder in the trial frame by yourself. To refine the cylinder axis, hold the cross cylinder with the handle parallel to the correcting cylinder axis. After each binary comparison, rotate the cylinder axis toward the preferred position of the red (or white) dots on the cross cylinder, depending on whether you are working with minus cylinder or plus cylinder trial lenses, respectively. For the next binary comparison, you will have to adjust the orientation of the cross cylinder handle to align with the new position of the trial cylinder axis. (This realignment of the cross cylinder takes place automatically in the phoropter.) To refine the cylinder power, hold the handle of the cross cylinder 45º away from the axis of the trial cylinder. This will align the lines between the red and white dots on the cross cylinder either parallel or perpendicular to the axis of the trial cylinder; then proceed as above.

Figure I-27 Hand-held Jackson cross cylinder. Notice the red and white dots that straddle the axis of the handle.

(Courtesy of Scott E. Brodie, MD.)