The phenomena which we are about to describe are, in several respects, different from those to which we have referred. They are seen in _monocular_ as well as in _binocular_ vision, and they are produced in all cases under a mental illusion, arising either from causes over which we have no control, or voluntarily created and maintained by the observer. The first notice of this class of optical illusion was given by Aguilonius in his work on optics, to which we have already had occasion to refer.[69] After proving that convex and concave surfaces appear plane when seen at a considerable distance, he shews that the same surfaces, when seen at a moderate distance, frequently appear what he calls _converse_, that is, the concave convex, and the convex concave. This conversion of forms, he says, is often seen in the globes or balls which are fixed on the walls of fortifications, and he ascribes the phenomena to the circumstance of the mind being imposed upon from not knowing in what direction the light falls upon the body. He states that a concavity differs from a convexity only in this respect, that if the shadow is on the same side as that from which the light comes it is a concavity, and if it is on the opposite side, it is a convexity. Aguilonius observes also, that in pictures imitating nature, a similar mistake is committed as to the form of surfaces. He supposes that a circle is drawn upon a table and shaded on one side so as to represent a convex or a concave surface. When this shaded circle is seen at a great distance, it appears a plane surface, notwithstanding the shadow on one side of it; but when we view it at a short distance, and suppose the light to come from the same side of it as the part not in shadow, the plane circle will appear to be a convexity, and if we suppose the light to come from the same side as the shaded part, the circle will appear to be a concavity. [69] See Chap. i. p. 15. More than half a century after the time of Aguilonius, a member of the Royal Society of London, at one of the meetings of that body, when looking at a guinea through a compound microscope which inverted the object, was surprised to see the head upon the coin depressed, while other members were not subject to this illusion. Dr. Philip Gmelin[70] of Wurtemberg, having learned from a friend, that when a common seal is viewed through a compound microscope, the depressed part of the seal appeared elevated, and the elevated part depressed, obtained the same result, and found, as Aguilonius did, that the effect was owing to the inversion of the shadow by the microscope. One person often saw the phenomena and another did not, and no effect was produced when a raised object was so placed between two windows as to be illuminated on all sides. [70] _Phil. Trans._ 1744.