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w. JAMES : duce. Two principal theories have been held on this matter, the " theory of identical points," and the " theory of projec- tion " each incompatible with the other, and each beyond certain limits becoming inconsistent with the facts. The theory of identical points starts from the truth that on both retinse an impression on the upper half makes us perceive an object as below, on the lower half as above, the horizon ; and on the right half an object to the left, on the left half one to the right, of the median line. Thus each quadrant of one retina corresponds as a whole to the similar quadrant of the other ; and within two similar quadrants, Fig. 1. al and ar, for example, there should, if the correspondence were consistently carried out, be geometrically similar points which, if impressed at the same time by light emitted from the same object, should cause that object to appear in the same direction to either eye. Experiment verifies this surmise. If we look at the starry vault with parallel eyes, the stars all seem single ; and the laws of perspective show that under the circumstances, the parallel light rays coming from each star must impinge on points within either retina which are geometrically similar to each other. The same result may be more artificially obtained. If we take two exactly similar pictures, smaller, or at least no larger, than those on an ordinary stereoscopic slide, and if we look at them as stereoscopic slides are looked at, that is, at one with each eye (a median partition confining the view of either eye to the picture opposite it), we shall see but one flat picture, all of whose parts appear sharp and single. 1 Identical points 1 Just so, a pair of spectacles held an inch or so from the eyes seem like one large median glass. The faculty of seeing stereoscopic slides single without an instrument, is of the utmost utility to the student of physio- logical optics, and persons with strong eyes can easily acquire it. The only difficulty lies in dissociating the degree of accommodation from the degree of convergence which it usually accompanies. If the right picture is focussed by the right eye, the left by_ the left eye, the optic axes must