OPTICAL ILLUSIONS OF MOTION. 



523 



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 con- 

 sists 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 complement- 

 ary 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 know- 

 ing at the time that Brewster had employed it,* 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. 



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 comple- 

 mentary 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, 



* The same experiment was also tried by my friend J. Aitken, Esq., of Darroch, Fal- 

 kirk, 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. 



