PHYSIOLOGICAL PARADOXES. 



object of moderate size at the opposite side 

 of the room. (Out of doors some equally well- 

 defined object at a distance, such as a flag- 

 staff, factory chimney, or church steeple, may 

 be taken.) While looking at the door-knob, 

 hold up in front of it the first finger of one 

 hand, pointing upwards, the rest of the hand 

 being closed. The door-knob is seen distinctly. 

 Less distinctly, but still quite plainly, are seen 

 two fingers, one on each side of the door- 

 knob. 



Now look directly at the finger, which is 

 only six or ten inches from the face. One 

 finger is seen distinctly. Less distinctly, but 

 quite plainly, one on each side of it, two door- 

 knobs are seen. 



Now for the explanation. 



We saw in the last section that the fovea 

 centralis in the macula lutea in the middle of 

 the retina at the back of the eye is the spot 

 of keenest vision. Consequently, the straight 

 line through this spot and the centre of the 

 lens is the line of direct vision, and that part 

 of the external world which is in this line is 

 the part most clearly seen. So that when we 

 wish to see any object clearly, each eye sets 

 itself so that its own line of direct vision points 

 to the object. Therefore the lines of vision 

 of the two eyes meet in the object. If the 

 object is at all near, these two lines enclose a 

 definite angle, and the two eyes are far from 



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