The effect of orientation on the clarity of Hermann grid illusory lines

To test the hypothesis that Hermann grid illusory lines are most clear when the grid is presented at a 45 deg, each of 20 participants underwent 10 trials in each of two conditions ('make the lines least clear' and 'make the lines most clear') which were run using a method of adjustment. A matched-pairs t-test applied to the means of the two conditions was significant beyond the 0.01 level. One-group t-tests supported the claim that the illusory lines are least clear for a vertical/horizontal orientation of the grid and most clear when it is rotated 45 deg. The results substantiate, in a systematic manner, 70 years of claims about the effects of grid orientation on illusory line clarity.