PHYSIOLOGICAL FACTOBS OF THE ATTENTION-PBOCESS. 477 continues to predominate so long as the accommodation of the right eye is maintained, so that save for momentary appearances of the whole of the vertical image, I see only images of the forms of figure 5, a and b. I have made many varieties of observations of this kind, using various patterns, of bright light (e.g., parallel bars of light forming a square area and presented in different positions successively to the two eyes), and always with similar results. 1 This observation seems to prove indisputably that con- traction of the intrinsic muscles of one eye sends up to the brain, presumably through afferent nerves of the muscles, an influence that directly re-enforces the activity of the cerebro- retinal tract of that eye, while it exerts no such effect upon the corresponding tract of the other eye, or exerts it in a minor degree only. 2 This re-enforcing effect of the activity of the intrinsic muscles of the eye may be demonstrated unmistakably in the case of direct visual sensations by studying the struggle of two visual fields of different colours. In studying the durations and rate of alternation of the states of consciousness during the struggle of two different visual fields and the changes of mode of apperception of ambiguous figures, it is necessary to make some objective record of the changes at the moment of their occurrence, because the general impression retained by the subject as to the relative frequency and duration of the different states of consciousness is altogether unreliable. So far as I am aware, two observers only have attempted systematically to record such changes, namely Dr. N. Lange 3 in the case of ambiguous figures, and Mr. Breese * in the case of the struggle of two different visual fields. Both these observers worked with two contact keys, the subject being instructed to press one key or the other according as 1 This observation seems to me to afford the most conclusive evidence of the separateness of the visual cortical areas for the two eyes, a fact of great importance for the theory of vision, because as I have pointed out in a previous paper (MiND, vol. x., p. 222) it seems to present an insuper- able difficulty to the theory of " Gegenfarben ". 2 In the recently published Monograph Supplement (vol. iv.) of the Psychological Review, Mr. E. B. Holt describes experiments which seem to show that, during voluntary movements of the eyes, visual sensations are interrupted or inhibited for a brief moment. I find in my notebook the following note : " It is noteworthy that in all cases the re-enforcing effect of activity of the eye-muscles does not seem to manifest itself until an appreciable period after the beginning of the effort ". There is there- fore nothing contradictory in my observations to those of Mr. Holt. 3 Phil. Stud., Bd. iv. 4 Psycholog. Review, Monograph Supplement, No. 11, 1899.
