(theoretical) ‘spectacles behind the eyes’ (Russell Hanson). They may decide that the prescription for those spectacles needs changing from time to time—science is corrigible—but without some such interpretative aid the practice of science would be impossible. The problems that this poses are at least as old as Galileo. His opponents suggested that the mountains he claimed to see on the Moon were distortions induced by his newfangled telescope. Experimental vision is clouded by the complexity of