The experiments were designed to investigate the optimum illusory effects of the various stimulus factors in Lipps figure (Fig. 1).Variables operated were as follows: (1) d′; distance between refracting lines X and Y (Fig. 6), (2) α, β; angle of refraction in X and Y (Fig. 2). (3) x′, y′; length of accessory lines attached to principal lines x and y, respectively (Fig. 2). (4) Spatial conditions of Lipps figure placed in vertical, horizontal or oblique directions, H, V, h, v, h′, f.Results obtained were as follows:A. The direction of the principal line x in the refracting line X of Lipps figure was seen relatively gently and the direction of y in Y steeply. But, in the absolute deflection, the direction of x in X alone was peen steeply as y in Y alone. After all, the positive illusory effects of Lipps figure were mainly due to the interaction between X and Y in XYX-or YXY-type arrangement (Exper. IX), B. Factors related of form:(1) The effect of the factor of the accessory lines presented the monotonic increasing curve in variation of x′ and the mountain-shaped curve in variation of y′. The maximum points appeared in x′/x=1 and y′/y or y′/d′=1/2 (variations; x′=0-7cm and y′=0-5cm. Exper. III and IV).(2) The effects of the factors of the refracting angles presented the mountain-shaped curves in variations of both α and β. The maximum points appeared in α=-20°--30° and β=15°-20° (variations; α=0°--75° and β=0°-90°. Exper. V, VI and VII).C. Factors related to spatial conditions:(3) The illusory effect of the factor of the distance between X and Y presented the monotonic increasing curve in variation of d′ (variations; d′=0-7cm. Exper. I and II).(4) The minimum illusory effect of the factor of the spatial rotation (anisotropy) of Lipps figure as a whole appered in H- or V-position, where the directions of principal lines coincided with the ordinate or the abscissa, and the maximum in h′- or f-position, where neither the direction of principal lines nor that of the other side-lines l and l′ forming a parallelogram together with them coincided with the ordinate or the abscissa (Exper. VIII).In conclusion, the specific effect in Lipps illusion of direction is caused by the interaction between different kinds of refracting lines X and Y the interaction regulated by the distance d′ and the interior angle φ of parallelogram (Fig. 6). However, the effects of other factors such as the length of accessory lines, the angle of refraction and the anisotropy can never be neglected.