Distortions of perception of the Müller-Lyer, Delboeuf, Oppel-Kundt and Zöllner type have been studied in psychophysical experiments conducted under computer control with a program of our own design arranging the stimuli, presenting them on the monitor, introducing alterations according to the subject’s command, recording responses, and handling the results. Our illusory figures designed by heteroluminant colours yielded the effects of perceived distortion qualitatively and quantitatively similar to the effects evoked by the white and black figures. Variations of luminance of colours of the figures and backgrounds within a relatively wide interval of its values did not evoke a noticeable change in strength of the perceived distortions, but if the luminance of the stimulus colour approached the magnitude which was determined as isoluminant with the background colour, the strength of the distortions changed significantly; it increased for the Brentano figure and decreased for the Delboeuf, Oppel-Kundt and Zöllner figures. The curves obtained for different subjects with different colour combinations were similar in shape, though horizontal parts and peaks of the curve were not at the same level. For isoluminant figures, the experimental curves demonstrating the strength variations of the perceived distortions in dependence of spatial parameters of the stimuli were similar in shape with the curves for heteroluminant figures, but lay above (Brentano) or below (Delboeuf... [to full text]