In the FCC example, which observation prompts the transition from plus to minus adjustments?

• A. Vertical lines darker
• B. Horizontal lines darker
• C. Lines equally dark
• D. No darkness observed

Answer: (A)

In this FCC (fused cross-cylinder) approach, you’re using a cross with perpendicular meridians and alternating plus and minus powers to pinpoint which meridian needs which kind of correction. The clue that tells you to switch from plus to minus is when the vertical lines become darker (sharper) with the current plus adjustment. That shows the vertical meridian is responding to the positive power, so to continue refining the prescription and balance both meridians, you move to minus to test the perpendicular (horizontal) meridian. If instead the horizontal lines darkened with plus, you’d interpret it as the meridian being helped by plus in that orientation and adjust accordingly. Lines being equally dark or no darkness observed don’t give a clear directive to switch.