The cross cylinder.
D000065:Accommodation, Ocular; D001251:Astigmatism / Q000175:diagnosis; D006801:Humans; D008722:Methods; D055095:Optics and Photonics / Q000295:instrumentation*; D011305:Presbyopia / Q000175:diagnosis; D012029:Refraction, Ocular / Q000295:instrumentation*; D012030:Refractive Errors / Q000175:diagnosis*