A Visual Illusion

Abstract

THE visual illusion described by Mr. Turner and by Prof. Boycott in NATURE of October 7 and 14 was described by me in NATURE of October 18, 1917 (vol. c., p. 126), and commented on by Dr. F. J. Alien and others on pp. 146, 165, 225, and 325 of the same volume. It had also been described in NATURE, vol. Ixx., p. 107, and vol. Ixxviii., pp. 225, 277, and 305. It was also pointed out by me in vol. c., p. 284, that the phenomenon had been fully described by Dr. John Aitken in the Journal of Anatomy and Physiology, vol. xiii., p. 322. The illusion may, perhaps, be best seen by looking through a microscope and slowly rotating the stage; as soon as the rotation is stopped the field appears to be revolving in the opposite direction, and so strong is the illusion that the stage may again be rotated very slowly in the original direction for 10° or 15° and will appear to the eye to be perfectly still. The same phenomenon may be seen when a pianola roll is stopped, the roll appearing to be slowly moving backwards. In some forms of pianola there is, in front of the record, a glass panel on which is a small knob for opening and shutting the panel; if the finger is placed lightly on this knob while the roll appears to be running back, I have the very curious tactual illusion that the knob is also moving upwards, and that it presses more and more against the finger. One or two others, however, with whom I have tried the experiment do not perceive the tactual illusion.