From Alex McDowell, London, UK
Catherine de Lange reports a virtual reality headset improving users' vision “to 20/30, which is pretty close to 20/20 vision” (4 August, p 4). In the UK, doctors and opticians now use a metric measure of visual acuity: “6/9 vision” means being able to read at 6 metres what one should, nominally, be able to read at 9 metres; “20/30” is the equivalent when distances are measured in feet.
