Those who have made quantitative studies in the optical illusion, especially in this particular illusion for open and filled spaces, have observed and commented on the instability of the illusion. Auerbach[11] says, in his investigation of the quantitative variations of the illusion, that concentration of attention diminishes the illusion. In the Zöllner figure, for instance, I have been able to notice the illusion fluctuate through a wide range, without eye-movements and without definitely attending to any point, during the fluctuation of the attention. My experiments with the tactual illusion have led me to the conclusion that it fluctuates even more than the optical illusion. Any deliberation in the judgment causes the apparent size of the filled space to shrink. The judgments that are given most rapidly and naïvely exhibit the strongest tendency to overestimation; and yet these judgments are so consistent as to exclude them from the category of guesses. [11] Auerbach, F., _Zeitsch. f. Psych. u. Phys. d. Sinnesorgane_, 1874, Bd. VII., S. 152. In most of my experiments, however, I did not insist on rapid and naïve judgments; but by a close observation of the subject as he was about to make a judgment I could tell quite plainly which judgments were spontaneous and which were deliberate. By keeping track of these with a system of marks, I was able to collect them in the end into groups representing fairly well the different degrees of attention. The illusion is always greatest for the group of spontaneous judgments, which points to the conclusion that all illusions, tactual as well as visual, are very largely a function of attention.