The author began by endeavouring to state with precision the nature of the difficulty which has been long felt on this subject. Two general facts are well ascertained, and present fair objects of inquiry, 1. That images formed on corresponding points of the retinæ of the human eyes, and on these only, naturally affect the mind in the same manner as a single image, formed on the retina of either eye; and, 2. That impressions made on different points of the retina of the eye, are naturally followed by inferences as to the relative position of the objects producing them, exactly opposite to those which follow impressions made on different points of the surface of the body.