Astigmats, who have been fully optically corrected, show an acuity configuration highly characteristic of the uncorrected astigmatism when they are tested with gratings of different orientations. This phenomenon, which is called meridional amblyopia, has been supposed to be neural in origin. This paper attempts to show that there is evidence for a critical period in the visual system of humans. In the experiment described here twelve astigmatic subjects were tested against a control group of six non-astigmatic (emmetropic) subjects. The astigmatic subjects differed in the ages at which they had been initially optically corrected. Square-wave gratings were used to test the subjects and measures of acuity for different orientations were recorded. It was found that those astigmatic subjects corrected above the age of seven showed the usual acuity decrement while those corrected below seven showed significantly smaller meridional amblyopia. The age of seven, therefore, was taken to mark the end of a critical period in the development of the human visual system, after which the system ceases to be structurally influenced by visual input.