90 PSYCHOLOGY. 



after whirling, we stop, we seem to be spinning in the reverse 

 direction for a few seconds, and then objects appear to con- 

 tinue whirling in the same direction in which, a moment 

 previous, our body actually whirled. The reason is that 

 our eyes normally tend to inaintoin their field of view. If we 

 suddenly turn our head leftwards it is hard to make the 

 eyes follow. They roll in their orbits rightwards, by a 

 sort of compensating inertia. Even though we falsely 

 think our head to be moving leftAvards, this consequence 

 occurs, and our eyes move rightwards — as may be observed 

 in any one with vertigo after whirling. As these move- 

 ments are unconscious, the retinal movement-feelings which 

 they occasion are naturally referred to the objects seen. 

 And the intermittent voluntary twitches of the eyes towards 

 the left, by which we ever and anon recover them from the 

 extreme rightward positions to which the reflex movement 

 brings them, simply confirm and intensify our impression 

 of a leftward-whirling field of view : we seem to ourselves 

 to be periodically pursuing and overtaking the objects in 

 their leftward flight. The whole phenomenon fades out 

 after a few seconds. And it often ceases if we voluntarily 

 fix our eyes upon a given point.* 



Optical vertigo, as these illusions of objective movement 

 are called, results sometimes from brain-trouble, intoxica- 

 tions, paralysis, etc. A man will awaken with a weakness 

 of one of his eye-muscles. An intended orbital rotation 

 will then not produce its expected result in the way of 

 retinal movement-feeling — whence false perceptions, of 

 which one of the most interesting cases will fall to be 

 discussed in later chapters. 



There is an illusion of movement of the opposite sort, 

 with which every one is familiar at raihvay stations. Habit- 

 ually, when we ourselves move forward, our entire field of 

 view glides backward over our retina. "When our move- 

 ment is due to that of the windowed carriage, car, or boat 



* The involuntary continuance of the eye's motions is not the only cause 

 of the false perception in these cases. There is also a true negative after- 

 image of the original retinal movement-sensations, as we shall eee in 

 Chapter XX. 



