History: The 100-year-old test for colour blindness

Did you know it was designed for military use?

If you’ve ever performed the ­Ishihara test for colour blindness, you’ll be familiar with the routine: you flick through page after page of colourfully concealed numbers while every so often throwing in the ‘nothing’ curveball, which your patient desperately tries to decipher like an inkblot at a psychiatrist’s office.