Testing visual acuity in children of preschool age has long been neglected in routine physical examinations. This failure may be due partly to the inability of children under school age and of those in kindergarten and in the first grade to read the letters of the Snellen chart and partly to a lack of confidence in the reliability of tests suited to children of these ages.
It seemed desirable, therefore, to find a test for visual acuity applicable to the child not yet able to read letters, to determine a set of standard conditions for testing which could be set up readily by the physician and to determine the reliability and validity of the test under standard conditions.
A simple test, suitable for our purposes, was described by Jackson.1 This is the "incomplete square" test of visual acuity. The test figure is 9 mm. square; it is filled in
WILDER RL, PETRIE KA, MARQUIS JL. AN EVALUATION OF THE INCOMPLETE SQUARE TEST OF VISUAL ACUITY FOR YOUNG CHILDREN. Am J Dis Child. 1935;50(5):1182-1186. doi:10.1001/archpedi.1935.01970110090012