The Holmes-Wright lantern was designed for carrying out tests of a seaman's ability to distinguish different coloured lights, for the Merchant Navy. The lantern was used to test a subject at a distance of 6 metres, mimicking the appearance of coloured lights 2 miles away. In general, this was achieved by viewing the lantern with a mirror set at about 3 metres away. The lantern has a revolving disc for changing the colour setting (to red, dark red, blue green, yellow green or white), an adjusting lever for large (5mm) or small (0.5mm) diameter lights, and a letter-box aperture for projecting the lights. The Holmes-Wright lantern was designed by W.D. Wright of Imperial College at the request of the Air Ministry in order to replace the Board of Trade lantern (see NAV0957). The model B was used until 2006/7. The model A lantern was used for the armed services and civil aviation tests.