Page:Mind (New Series) Volume 15.djvu/352

 338 W. MCDOUGALL : in intensity. Other similar conditions have yielded similar results of cerebral induction of sensation, but it is more readily obtained in the case of after-images, as in the following cases. In a figure like figure 2 a central square area compris- ing nine white discs was blocked out with black ink. Under brilliant illumination a bright positive after-image was ob- tained, and I concentrated my attention upon the grouping of the discs, causing them to change repeatedly from one grouping to another. When I then looked for the central gap in the after-image corresponding to the black square at the centre of the figure it was no longer visible, the rows of white discs ran continuously across the centre. Again about the centre of a large black card a number of small square holes were cut, separated from one another by narrow strips and arranged in vertical and horizontal rows. This, held against a white background appears like figure 10. l FIG. 10. It will be noticed that, just as in the case of figure 2, the white areas tend to appear grouped alternately in vertical or hori- zontal rows and in other groupings, though less insistently than in the case of figure 2. This figure fixated against a 1 In connexion with this figure I may take the opportunity to point out a curious illusory appearance presented by it, which, so far as I know, has not previously been described. If it is held in a good light at some 40 centimetres from the face and the eyes are allowed to wander over it, one sees a white or grey spot on each junction of the black bars except the one directly looked at. If the eyes are kept steadily fixated upon any one point the white spots rapidly become dimmer until after a few seconds they disappear. I have not been able to discover the explana- tion of this appearance. That the white spot does not appear upon the junction directly looked at (i.e. the one whose optical image falls upon the fovea) proves that it is not a simple contrast-effect and suggests that it is in some way due to the rod-apparatus. This effect may be observed particularly well when a window with small leaded panes is looked at against a bright sky. Each junction of the cross-bars, except the one at the centre of the field, appears to be perforated with a small hole.