Our instrumentarium is, of course, in first line, the collection of apparatus bought and constructed through the fourteen years of work. Yet with the new expansion of the institute a considerable number of psychological, physical, and physiological well-tested instruments has been added. Especially in the departments of kymographic, chronoscopic, and optical apparatus the equipment presents a satisfactory completeness; its total value may be estimated to represent about twelve thousand dollars. Yet the place of the laboratory which we appreciate most highly is not the instrument-room but the workshop, in which every new experimental idea can find at once its technical shape and form. Whether those experimental ideas will be original and productive, whether their elaboration will be helpful for the progress of our young science, in short, whether the work in the new laboratory will fulfil the hopes with which we entered it, may be better decided as soon as a few further volumes of the Harvard Psychological Studies shall have followed the present one, which is still from cover to cover a product of Harvard's pre-Emerson-Hall period. OPTICAL STUDIES STEREOSCOPIC VISION AND THE DIFFERENCE OF RETINAL IMAGES BY G. V. HAMILTON The question which the Laboratory proposed to me for experimental enquiry was one which demanded a definite reply of yes or no. The positive answer seemed a necessary consequence of the traditional psycho-physiological theories, while a certain practical consideration seemed to suggest the negative solution. The question which seems to have been overlooked so far was this: According to the theory of stereoscopic vision two points of light which are seen by each of the two eyes under the same angle appear to lie in the same plane; as soon as the angle for the right eye is larger than that for the left, that is, as soon as the two stimulated retinal points in the right eye are more distant than the two retinal points stimulated in the left eye, the right light-point seems to be farther away than the left one. If we relate them to planes vertical on the ideal binocular fixation-line, the right point lies in a more distant plane. This principle, which, of course, controls all arrangements for stereoscopic effect, is deduced from experiences in which the fixation-line is vertical to the line that connects the nodal points of the two eyes; the plane in which the equally distant points lie is then parallel to the forehead. If, on the other hand, the eyes are turned to the side, that is, if the ideal fixation-line forms an acute angle with the line connecting the eyeballs, the two fixated light-points, which lie in a plane perpendicular to the fixation-line, cannot be seen by the two eyes under the same angle. Any object on my right side is somewhat nearer to my right eye than to my left, and therefore must throw a larger image on my right retina. The two light-points of a plane vertical