THE BLIND SPOT. 



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moving the paper to and fro at a certain distance both black 

 dots will disappear. 



(c.) Close one eye, and fix the point a (Fig. 126); on 

 moving the paper a certain distance (about 16 cm.), one sees 

 a complete cross, and to most observers the horizontal bar 

 appears uppermost. 



Fig. 126. 



2. Map out the Blind Spot. 



(a.} Make a cross on the centre of a sheet of white paper, 

 and place it on a table about ten or twelve inches from you. 

 Close the left eye, and look steadily at the cross with the 

 right eye. Wrap a penholder in white paper, leaving only 

 the tip of the pen-point projecting, dip the latter in ink, 

 or dip the point of a white feather in ink, and keeping the 

 head steady and the axis of vision fixed, place the pen-point 

 near the cross, and gradually move it to the right until the 

 black becomes invisible. Mark this spot. Carry the black- 

 ened point still further outwards until it becomes visible 

 again. Mark this outer limit. These two points give the 

 outer and inner limits of the blind spot. Begin again, 

 moving the pencil first in an upward and then in a down- 

 ward direction, in each case marking where the pencil 

 becomes invisible. If this be done in several diameters, an 

 outline of the blind spot is obtained, even little prominences 

 showing the retinal vessels being indicated. 



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