Four-colour vision research. Research subject using a joystick during a test for four-colour vision or tetrachromacy. This is a rare genetic mutation (on the X chromosome) that would form an extra type of cone cell on the retina. Only female tetrachromats are possible. Beams of light are used to test whether potential tetrachromats can perceive different colours to those normally seen. A mixture of green and red light appears yellow to trichromats, but to a tetrachromat, the mix may look different to a comparison beam of yellow light. Photographed at the University of Newcastle-upon-Tyne, England, in 2004.