Four-colour vision research. Research subject (upper left) undergoing 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. The beams of light are used to test whether potential tetrachromats can perceive different colours to those normally seen. A mixture of green (near side) and red (centre) light appears yellow to trichromats, but the mix may look different to the comparison yellow light (far side) to a tetrachromat. Photographed at the University of Newcastle-upon-Tyne, England, 2004.