PHYSIOLOGICAL FACTOBS OF THE ATTENTION-PROCESS. 475 reflexly or habitually drawn to those features of the visual field that move relatively to the retinae, because those that are due to objects or stimuli residing wholly within the eye and moving with it, such as musca volitantes and after-images, have no value as signs of objective existences and are there- fore habitually neglected. This view of the matter is borne out by the fact that as one accustoms oneself to pay atten- tion to these appearances they assume a much more stable character. Herr Wirth 1 takes a similar view of the sudden comings and goings of after-images and argues that the chief determining influences are ' Apperceptive Momente '. Prof. Hering 2 too maintains that the disappearance of a projected after-image on movement of the eyes is due to the attention being drawn to new features of the background, and he shows that when an after-image is observed in complete darkness eye-movements do not cause it to disappear, and that the more homogeneous the surface upon which an after-image is projected the less does the after-image tend to disappear on movement of the eyes. My own observations on the influence of the surface of projection are entirely in agreement with Prof. Bering's, and it is therefore not necessary to describe them here, but in regard to the influence of eye-movements upon after- images I think we may go farther than Hering, and say that, so long as after-images are observed in total darkness, eye-movements not only do not tend to cause them to dis- appear, but tend rather to maintain them and to restore them to consciousness when they have disappeared. Lateral movements of the eyes in the dark will frequently cause the reappearance of an after-image, but the effect is feeble and inconstant. A movement of convergence with accommoda- tion exerts a much more powerful and unmistakable influence of this sort, and it is possible to restore an after-image several times by making repeated efforts of convergence, the after-image reappearing at each effort and persisting for some few seconds. I find that the form of after-image best suited for this kind of experiment is that last stage of the after-image of a bright light which appears, as noted in a previous paper, 3 as a fuzzy ill-defined grey or dull white, and which, as I there suggested (a suggestion which further experience has confirmed), is due to the apparatus for vision in a dim light (the rod-apparatus or Dunkel-Aparat of v. Kries). 1 Philosoph. Stud., Bd. xvi. 2 Von Graefe's Archiven, Bd. xxxvii., and Zeitschrift f. Psychologie, Bd. i. 3 MIND, vol. x., p. 242.
