Answer:
(1) the lighting, color, character, and disposition of objects; (2) the mechanism by which the image is formed upon the retina; (3) various optical defects of this mechanism; (4) the sensitiveness of the parts of the retina to light and color; (5) the structure of the retina; (6) the parts played by monocular and binocular vision; and (7) the various events which follow the formation of the image upon the retina, all help create the effect of an optical illusion.