On the non-additivity of visual tilt illusions

In Experiment I, the visual tilt illusion induced by two orthogonal lines was found to differ from the sum of the illusions induced by each line independently. The length of one of the orthogonal-inducing lines was varied in Experiment II with the other inducing line remaining a fixed length. As length increased from zero (no line) to 100%, the size of the illusion increased monotonically. The increase in illusion was not due to the increasing length of the variable line alone since, in the absence of the line of fixed length, increasing the length of the variable line produced no significant trend in the illusion (Experiment III).