In a recent communication1 I briefly described a new phenomenon in stereoscopic vision as follows :
If two nearly vertical black lines on a white background, at a slight angle to each other, are "fused"2 by means of a stereoscope, they will ordinarily appear as a single intermediate line which has a fore and aft inclination. Let us assume that the upper end of the line before the right eye is tilted towards the right, and that of the other line towards the left. If now a smoked glass is placed before the right eye, the "fused" line will swing slightly towards the left, producing also a diminution in the stereoscopic effect. If the smoked glass is placed so as to obscure only half of the line, the "fused" line will appear bent in the middle. If instead of using a smoked glass, a graduated shadow is thrown on