IT IS generally recognized that cyclofusional movements of the two eyes can occur in the interest of maintaining single binocular vision, especially in the unnatural and the forced optical conditions that, for example, can be produced in the stereoscope. A historical survey shows that this recognition has come not without controversy. Nagel,1 it seems, was the first to point out that a cyclotorsional movement took place when horizontal lines, observed in the stereoscope, were rotated in opposite directions. Independently, von Helmholtz2 found the same evidence with an arrangement of prisms when rotating the images in the two eyes in opposite directions. At the same time, Hering,3 while not denying the possibility of such movements, was unconvinced by the experiments of Nagel and von Helmholtz, but more especially by his own attempts to find such movements with a stereoscope. Only later, after Nagel had repeated both the experiments