III. _An Optical Illusion._--In the sphere of vision many very interesting facts are constantly coming to light. Sight is the most complex of the senses, the most easily deranged, and, withal, the most necessary to our normal existence. The report of the following experimental study will have the greater utility, since, apart from any intrinsic novelty or importance the results may prove to have, it shows some of the general bearings of the facts of vision in relation to AEsthetics, to the theory of Illusions, and to the function of Judgment. Illusion of the senses is due either to purely physiological causes or to the operation of the principle of Assimilation, which has already been remarked upon. In the latter case it illustrates the fact that at any time there is a general disposition of the mind to look upon a thing under certain forms, patterns, etc., to which it has grown accustomed; and to do this it is led sometimes to distort what it sees or hears unconsciously to itself. So it falls into errors of judgment through the trap which is set by its own manner of working. Nowhere is the matter better illustrated than in the sphere of vision. The number of illusions of vision is remarkable. We are constantly taking shapes and forms for something slightly different from what, by measurement, we actually find them to be. And psychologists are attempting--with rather poor success so far--to find some general principles of the mechanism of vision which will account for the great variety of its illusions. Among these principles one is known as Contrast. It is hardly a principle as yet. It is rather a word used to cover all illusions which spring up when surfaces of different sizes and shapes, looked at together or successively, are misjudged with reference to one another. Wishing to investigate this in a simple way, the following experiment was planned and carried out by Mr. B.