THE PERCEPTION OF THINGS. 89 



ever we think that our eves move, but fail to get the retinal 

 movement-feeling. We believe objects to be still, on the 

 contrary, 1) whenever we get the retinal movement-feeling, 

 but think jur eyes are moving ; and 2) whenever we neither 

 think our eyes are moving, nor get the retinal movement- 

 feeling. Thus the perception of the object's state of motion 

 or rest depends on the notion we frame of our own eye's 

 movement. Now many sorts of stimulation make our e^es 

 move without our knowing it. If we look at a waterfall, 

 river, railroad train, or any body which continuously passes 

 in front of us in the same direction, it carries our eyes with 

 it. This movement can be noticed in our ej-es by a by- 

 stander. If the object keep passing towards our left, our 

 eyes keep following whatever moving bit of it may have 

 caught their attention at first, until that bit disappears 

 from view. Then they jerk back to the right again, and 

 catch a new bit, which again they follow to the left, and so 

 on indefinitely. This gives them an oscillating demeanor, 

 slow involuntary rotations leftward alternating with rapid 

 voluntary jerks rightward. But the oscillations continue for 

 a while after the object has come to a standstill, or the 

 eyes Are carried to a new object, and this produces the illu- 

 sion that things now move in the opposite direction. For 

 we are unaware of the slow leftward automatic movements 

 of our eyeballs, and think that the retinal movement-sen- 

 sations thereby aroused must be due to a rightward motion 

 of the object seen ; whilst the rapid voluntary rightward 

 movements of our eyeballs we interpret as attempts to pur- 

 sue and catch again those parts of the object which have 

 been slipping away to the left. 



Exactly similar oscillations of the eyeballs are produced 

 in giddiness, with exatly similar results. Giddiness is easi- 

 est produced by whirling on our heels. It is a feeling of 

 the movement of our own head and body through space, 

 and is now pretty well understood to be due to the irrita- 

 tion of the semi-circular canals of the inner ear.* When, 



* Purkinje, Mach, and Breuer are the authors to whom we mainly owe 

 the explanation of the feeling of vertigo. I have found (American Jour- 

 nal of Otology. Oct. 1882) that in deaf-mutes (whose semi-circular canals 

 or entire auditory nerves must often be disorganized) there very frequently 

 exists no susceptibility to giddiness or whirling. 



