606 PSYCHOLOGY. 



Driven thus from the body at large, where next shall the 

 circumstantial evidence for the feeling of innervation lodge 

 itself? Where but in the muscles of the eye, from which 

 small retreat it judges itself inexpugnable. Nevertheless, 

 that fastness too must fall, and by the lightest of bom- 

 bardments. But, before trying the bombardment, let us 

 recall our general principles about optical vertigo, or illu- 

 sory appearance of movement in objects. 



We judge that an object moves under two distinct sets 

 of circumstances : 



1. When its image moves on the retina, and we know 

 that the eye is still. 



2. When its image is stationary on the retina, and we 

 know that the eye is moving. In this case we feel that we 

 folloio the object. 



In either of these cases a mistaken judgment about the 

 state of the eye will produce optical vertigo. 



If in case 1 we think our eye is still when it is really 

 moving, we get a movement of the retinal image which 

 we judge to be due to a real outward motion of the 

 object. This is what happens after looking at rushing 

 water, or through the windows of a moving railroad car, or 

 after turning on one's heel to giddiness. The eyes, without 

 our intending to move them, go through a series of invol- 

 untary rotations, continuing those they were previously 

 obliged to make to keep objects in view. If the objects had 

 been whirling by to our right, our eyes when turned to 

 stationary objects will still move slowly towards the right. 

 The retinal image upon them will then move like that of an 

 object passing to the left. We then try to catch it by vol- 

 untarily and rapidly rotating the eyes to the left, when the 

 involuntary impulse again rotates the eyes to the right, con- 

 tinuing the apparent motion ; and so the game goes on. 

 (See above, pp. 89-91.) 



If in case 2 we think our eyes moving when they are in 

 reality still, we shall judge that we are following a moving 

 object when we are but fixating a steadfast one. Illusions 

 of this kind occur after sudden and complete paralysis of 

 special eye muscles, and the partisans of feelings of efferent 



