520 THE POPULAR SCIENCE MONTHLY. 



find that our simplest conceptions of angular or apparent magnitude 

 are very closely bound up with, if not directly due to, the sensations 

 of muscular fatigue in moving the eyeball or head so as to bring the 

 successive parts of the object into the center of vision. 



Hence, although optical illusions are of many diverse kinds illu- 

 sions of color, illusions of form, illusions of size, illusions of distance, 

 illusions of solidity, and illusions of motion they have all to be con- 

 sidered from the twofold standpoint, the purely optical and the psy- 

 chological. 



For some months the writer of this article was engaged upon a study 

 of one set of optical illusions, namely, the illusions of motion, and a 

 number of observations, collected at intervals over several years, have 

 been added by him to the stock of knowledge previously gleaned by 

 Brewster, Wheatstone, Faraday, Plateau, and others. Brewster made 

 a number of observations, in the early days of railways, on the various 

 illusions which can be found by watching objects from a moving train ; 

 Wheatstone investigated a curious case of apparent fluttering motion 

 at the border of two brightly illuminated colored surfaces due prob- 

 ably to the attempt of the unachromatic eye to obtain fruitlessly a 

 distinct focus of the border-line between the unequally refrangible 

 colors known as the illusion of the " Fluttering Hearts " ; Faraday 

 investigated the illusions produced by intermittent views of moving 

 objects, since developed in the phenakistiscope and zoetrope, and 

 kindred toys, and due to persistence of visual impressions. Brewster, 

 moreover, drew attention to the existence of another class of illusions 

 illusions of subjective complementaiy motion the typical case of which 

 occurs also in railway-traveling. After looking out of the window 

 at the pebbles and other objects lying beside the line, as they pass be- 

 fore the eyes, let the eyes be closed suddenly, when there will at once 

 be perceived an apparent motion in the opposite sense, undistinguish- 

 able forms and patches of light seeming to rush past the blank field. 

 This was recorded by Sir David in 1848, and the phenomenon was 

 referred by him to a subjective complementary motion going on simul- 

 taneously, and so causing a compensation of the impressions moving 

 over the retina. A kindred phenomenon had been even earlier noted 

 by R. Addams, who, in 1834, narrated how, after looking for some 

 time at a waterfall and then at the water- worn rocks immediately con- 

 tiguous, he saw the rocky surface as if in motion upward with an 

 apparent velocity equal to that of the descending water. This he 

 ascribed to an unconscious slipping of the inferior and superior recti 

 muscles of the eyeballs, which he thought occurred while watching 

 the falling water, and which he supposed to continue unconsciously 

 after the gaze had been transferred to stationary objects. This expla- 

 nation differs from the one offered by Brewster, namely, that there 

 was a subjective opposite movement going on simultaneously, so caus- 

 ing a compensation of the impressions moving over the retina. Brew- 



