Page:Popular Science Monthly Volume 18.djvu/539

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eye is subsequently directed to a stationary object it continues the habit of thus oscillating, causing the observer to attribute to the object a velocity of opposite sign to that just observed. M. Javal alleges in support of this view the appearance presented in the ophthalmoscope of the retina of a person affected with nystagmus. This affection consists in continual rapid involuntary movements to and fro of the eye. The retina, under these circumstances, appears to be animated with a vibratory motion which M. Javal declares to be identical in character with the apparent movements of the circles. In another place, M. Javal has endeavored to prove that the interior and exterior recti muscles of the eyeball are more prone to this slipping than are the superior and inferior recti, and that these illusions of complementary motion succeed better for motions in an horizontal sense than for vertical and oblique motions. My own experience, and that of other observers, admits of no such conclusion being drawn.

An experiment of Brewster's, which the writer tried without knowing at the time that Brewster had employed it,[1] has an important bearing on the muscular-slipping theory. A disk marked out into black and white sectors, as in Fig. 3, was caused to rotate at about one revolution per second, so that the separate sensations of black and white were not confused. The eye was steadily directed for twenty or thirty seconds at the central point, and then the gaze was suddenly turned upon some fixed objects, or at a distant landscape. For two

Fig. 3. Fig. 4.
PSM V18 D539 Eye muscle slipping test wheels.jpg

or three seconds a hazy rotation is noticed at the center of the field of vision. Now, if the muscular-slipping theory holds good, the complementary movement of rotation must be due to a slipping of the whole of the muscles of the eyeball, and would affect objects all over the field of vision with an equal angular velocity. This is not the case, the apparent complementary rotation being confined to the central field,

  1. The same experiment was also tried by my friend J. Aitken, Esq., of Darroch, Falkirk, who independently observed the phenomenon described by Addams, and who has also communicated to the Royal Society of Edinburgh a number of experiments on kindred illusions.