 In after-images only the remains of the form or species caused by the first act of perception are seen. But the belief that the principal object is seen when after-images are present is not caused by the first vision or even by the remains of the species imprinted during vision. It is caused by strong imagination which leads the observer to judge falsely that what he sees in an after-image is the same as what he saw when the principal object was presented (Wood 1982, 228).