It is commonly supposed that the speed of the slow phases of normal optic nystagmus varies with the speed of visual objects. The accuracy and limits of that correlation are, however, unknown. As we pointed out in the first paper of this series, the slow phase represents a kind of optic pursuit of moving objects and might be called with propriety the pursuit phase.
For the purposes of this discussion the pursuit may be called adequate when it is sufficiently accurate to permit clear vision. Whenever the line of regard lags behind or overshoots the moving object, vision became more or less blurred and the pursuit may be called inadequate. Determination of the degree of adequacy with which normal eyes pursue at various velocities of the visual object seems important, not only to provide a base line for the study of abnormalities, but also for an understanding of the mechanism