The results were absolutely uniform (the author alone as subject); the after-image streak always lay on that side of the moving light _toward_ which the _slow_ eye-movements were directed, that is, the lamp appeared to drift obliquely up or down and in a lateral direction _opposite_ to that of the slow eye-movements. Apart from its vertical displacement, then, the lamp behaved like the less intensely illumined parts of the visual field, seeming to be totally invisible during the swifter eye-movements. Now since the experiment was done in a partially darkened room and the eyes were partly adapted to darkness, the lamp should have been intense enough adequately to stimulate the retina even during the more rapid movements, and might be expected to leave an after-image streak on that side toward which these rapid movements were directed, and differing only from the streaks seen during the slow eye-movements in being inclined at a less angle from the horizontal. Yet no such streaks were visible. These observations were made at about the same number of seconds after the rotation stopped, as the photographs were taken that are recorded in the preceding paper of this volume. The rapid movements were therefore about one sixth as long in duration as the slower ones. Since the respective amplitudes of rapid and slow must average very nearly the same, the rapid movements must have been about six times as swift as the slow movements. It needs therefore to be shown beyond a doubt that the 2 c. p. lamp _was_ bright enough, in view of the briefness of stimulation of any one retinal element during the rapid eye-movement, to be above the threshold of perception. For this reason the experiment was not continued with other subjects. The certainly adequate degree of illumination was realized during the photography of the eyes described in the preceding paper. Here during the post-rotary dizziness an arc lamp (of 6 amp.) was in front of the face and but a little to one side of the primary line of regard; it was 60 cm. distant from the eyes and on a level with them; a lens condensed the rays on the two eyes, and the light was diminished only just enough as not to be painful, by a dilute screen of copper ammonium sulphate about 3 cm. thick. Of course such an illumination must adequately stimulate each retinal element even during the most rapid eye-movements. Nevertheless with the four subjects that were photographed the arc lamp, like the rest of the visual field, seemed always to swim in one direction, and that opposite to the slower eye-movements. In one case where the eyes were photographed without the adiathermal screen, and the light was rather painfully intense, the lamp was still seen to drift in one and the same direction. There was never any trace of its moving to and fro, as there should have been had it been visible during both phases of the nystagmiform movements. [Illustration: Fig. 1]