THE EYE AND THE PHYSIOLOGY OF VISION. 585 



Third. By determining the amount of motion neces- 

 sary to move the view of the eyeballs from one point to an- 

 other we make inferences as to the distance absolute and 

 relative of the objects seen. If an individual closes one 

 eye and then with his arm and finger extended forward, 

 moves towards the wall with the intention of halting when 

 his finger is, say just an inch from the wall, he will find 

 himself quite unable to do this satisfactorily, sometimes 

 touching the wall while he thinks he is still several inches 

 away, at other times suspecting the distance from his finger 

 tip to the wall to be about an inch, when it is five or six times 

 that amount. His inability to form correct judgments of dis- 

 tance in this case arises from the lack of using both eyes. 



OPTICAL ILLUSIONS. 



It has been pointed out that the eye is enabled to give 

 us sensations of color only. All knowledge of form, size, 

 distance, solidity, etc., are inferences made by the mind. 

 One would naturally suspect, therefore, that these infer- 

 ences would occasionally prove faulty and so give rise to il- 

 lusions, a deception which we then attribute to our physical 

 senses instead of holding our own minds responsible for 

 them. 



Several of the more striking forms of illusions are men- 

 tioned here. 



1. Irradiation. A light area on a dark background 

 seems larger than a black area on a black background, 

 even when these areas are mathematically alike. A hole 

 through an object, such as a wall, or even a piece of card- 

 board, seems larger than the solid piece that entirely fills 

 the hole. A person in light attire seems larger than in dark 

 attire. The explanation of this lies in the lack of sharp 

 focussing in the eye. When the focus of an object does not 

 fall directly on the retina, but the rays of light strike the 

 retina before meeting at a point, it is evident that a larger 

 area of the retina is affected. Turning to Figure 184 it is 

 readily seen that the image of the point a is a correspond- 



