It is a familiar fact that when the head is passively turned about its vertical axis, the eyes do not move with the head but lag behind, keeping their fixation on that object toward which they were directed before the head moved. The eyes move in their sockets in a direction opposite to that in which the head has moved. Now it has been proved beyond a doubt by the experiments of Mach,[2] Crum Brown,[3] and Breuer,[4] that these lagging movements of the eyes are reflex and are governed by the semi-circular canals, which are stimulated directly by the motion of the head. Similar reflex eye-movements are found when the head is turned about some other than its vertical axis, the direction of such movements being always in confirmation of the theory. All these movements, together with the theory, are well described in the summaries of Peters[5] and Nagel.[6] The present paper deals solely with the eye-movements that occur after rotation of the head about its vertical axis. The mechanism of these lagging, reflex movements is not, then, identical with that which enables us, when the head is at rest, to fix on and follow a luminous moving object,--the "pursuit movements" of Dodge.[7] It is, however, identical with that of Dodge's "fourth type"[8] and that of the compensatory eye-movements described by Brown,[9] Nagel,[10] and Delage,[11] and recently studied by Angier.[12] This function of the semi-circular canals was first suggested by Goltz in 1870. Now if the rotary movement of the head is prolonged, the eyes lag for a while on their first fixation-point, and then dart suddenly forward to a new fixation-point on which they rest for a while as before, until they dart forward again. Therefore if the head continues to rotate, the eyes fall into a regular and well-marked nystagmus. In this the lagging movements, or those opposite to the direction of the head, are called "compensatory," and are relatively slow and long. Their rate coincides closely if not exactly with that of the head-movement. But the movements forward, in the direction of the head-movement, are short and swift. Such are the facts during the rotation of the head. But if this rotation has been somewhat prolonged, the ocular nystagmus continues after the head and body are brought to rest. But now its phases are reversed, and the slower eye-movements are in that direction in which the head has moved; while the swifter are in what before was the lagging direction. These observations are in accord with the semicircular canal theory, and are well established by various investigators.[13] This paper presents the results of a photographic study of the reflex eye-movements following after rotation of the head (and body) about the vertical axis.