Judged: two independent readers; copy check clear.
optical defect
optical defect In this historical passage, an optical defect means an apparent problem in vision thought to underlie a visual disturbance.
In this historical passage, an optical defect means an apparent problem in vision thought to underlie a visual disturbance. It refers here to seeing numerous dark dots or spots against a white surface, interpreted by the writer as a form of scattered blind areas in the visual field.