Four-colour vision research. Researcher (upper right) watching a woman (lower right) 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 investigate whether potential tetrachromats can perceive different colours to those normally seen. A mixture of red and green light appears yellow to (normal) trichromats, but it may look different when seen by a tetrachromat. Photographed at the University of Newcastle-upon-Tyne, England, in 2004.