Variant of the Lancaster diplopia test.

THE Lancaster diplopia test (Lancaster, 1939) is simple and sensitive, shows topical cyclophoria easily, and is applicable to alternators with abnorinal retinal correspondence, so that in these respects it must be regarded as superior to the Hess test (Hess, 1916). The Lancaster method of recording, however, is less lucid, and attempts have been made, for example by Tulloch (1950) and Hugonnier (1952), to combine the two tests. Tulloch projected light from a green Lancaster torch on to a Hess background studded with red lights, reversing the red and green goggles to record secondary deviation. After further experiments to improve the method of recording, we have devised an even simpler background consisting of a grey Essex board, 39" x 39", on which is marked in black a central spot surrounded by a black square at 1" distance (i.e. with the centres of each side of the square at 5j-" from the spot, representing a 15° excursion of the eye) and another at 28" (equivalent approximately to 300). The Lancaster test with two torches is so sensitive that one can eliminate the small squares of the Hess chart without loss of " significant form ".