Page:Mind (New Series) Volume 12.djvu/490

 476 W. MCDOUGALL : But the following experiment demonstrates this re-enforc- ing effect of activity of the eye-muscles more satisfactorily than the observation of a simple after-image, and at the same time it shows that the intrinsic muscles of the eye play the chief part. By two successive applications of ^ grain of atropine sulphate to the left eye I completely paralysed the ciliary muscle and the iris. 1 I then held a slip of white, card (about 20 mm. by 50 mm.) in the path FIG. 4. of a ray of direct sunlight entering the dark room. I first held it in the vertical position and fixated a point at the middle of the bright surface for 30" with the left eye alone, and then held it in the horizontal position and fixated the same point for 30" with the right eye alone. With head covered and eyes closed and in the primary position of rest I then observed the after-images. The two bright positive after-images struggle for predominance just like FIG. 5. vertical and horizontal white strips combined in a stereo- scope, appearing alternately like figure 4, a and b, the vertical strip predominating on the whole because the larger pupil of the left eye has admitted the more light. On then making an effort of accommodation the horizontal after- image always predominates at once, usually to the total exclusion of the vertical after-image of the left eye, and 1 The atropine paralyses the motor nerve-endings in the muscles, and so prevents impulses passing down the nerves from affecting the muscles.