xxvn.] VISION. 277 



smaller when the eye is ficcommocjated for a 

 nearer object. 



4. Prick in a card two small holes close together, 

 so that both are within the diameter of the 

 pupil. Holding the card before one eye (the 

 other being closed) with the holes horizontal, 

 hold a fine needle vertically at a distance of 

 twelve inches. It will be seen distinctly as one 

 when the eye is accommodated for it, but double 

 when accommodation is made either for farther 

 or nearer objects. Both the double images will 

 be dimmer than the single one. 

 When a double image appears and 



a. The eye is adjusted for far objects ; stopping 

 the right-hand hole will make the left-hand 

 image disappear. 



b. The eye is adjusted for near objects ; stopping 

 the right-hand hole will make the right-hand 

 image disappear, 



When the eye is exactly accommodated for 

 the object, the rays passing through the two 

 holes are united at the focus on the retina. 

 In a, the rays fall upon the retina before 

 they are so united. Stopping the right-hand 

 bole removes the image on the right-hand 

 side of the retina. But images on the right- 

 hand side of the retina, are referred by us 

 to objects on our left-hand side, hence it is 

 the left-hand image which disappears. 

 In b, the rays are united, and cross at the 



