An Optical Illusion

Abstract

THE optical illusion described in NATURE, vol. xxiv. p. 54, is, as I have already mentioned, referred to by Priestley (History, &c, Vision, Light, and Colours, vol. ii. p. 725). The description is as follows:—“M. Le Cat well explains a remarkable deception by which a person shall imagine an object to be on the opposite side of a board when it is not so, and also inverted and magnified. It is illustrated by Fig. 162, in which D represents the eye and C B a large black board pierced with a small hole. E is a large white board placed beyond it, and strongly illuminated, and d a pin or other small object held betwixt the eye and the first board. In these circumstances the pin shall be imagined to be at F on the other side of the board, where it will appear inverted and magnified, because what is in fact perceived is the shadow of the pin upon the retina; and the light that is stopped by the upper part of the pin, coming from the lower part of the enlightened board, and that which is stopped by the lower part coming from the upper part of the board, the shadow must necessarily be inverted with respect to the object”. (“Traité des Sens”, par M. Le Cat, Amsterdam, 1744, p. 298.)