Most commuters keep their eyes down in the morning while traipsing through the gleaming corridors of London’s Euston Underground station. Their attention is focused on the feet, the coffee or, beyond that, the rigours of the day into which they hurtle. If any did manage to bravely look up, however, they’d catch a glimpse of a new addition to this, Britain’s sixth busiest railway station: a phalanx of tiny white boxes stuck fast to the ceiling, one every ten or so paces. These are Bluetooth beacons, transmitters able to provide a GPS lock-on to a phone or tablet even here, deep in London’s soil. Each one is able to produce a tiny miracle: the ability for a blind or partially sighted person to navigate the station’s warren of corridors with precision, without help, using nothing but a mobile phone.

It works eerily well. On a sweltering September afternoon, I’m told to put on a pair of glasses. They look like something designed by Jackson Pollock: chicly black-framed, lenses splattered with thousands of tiny droplets of paint swarmed around the centre. It’s possible to see through them, particularly at the periphery, but only in sketchy patches. This is what, I’m told, it is like to be visually impaired.