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EXPERIMENTAL PHYSIOLOGY. 



4. Imperfect Judgment of Distance. 



(a.) Close one eye, and hold the left forefinger vertically 

 in front of the other eye, and try to strike it with the right 

 forefinger. On the first trial one will probably fall short of 

 the mark, and fail to touch it. Close one eye, and rapidly 

 try to dip a pen into an inkstand, or put a finger into the 

 mouth of a bottle placed at a convenient distance. In both 

 cases one will not succeed at first. In these cases one loses 

 the impressions produced by the convergence of the optic 

 axes, which are important factors in judging of distance. 



(b.) Hold a pencil vertically about 15 cm. from the nose, 

 fix it with both eyes, close the left eye, and then hold the 



Fig. 135. 



right index finger vertically, so as to cover the lower part 

 of the pencil. With a sudden move try to strike the pencil 

 with the finger. In every case one misses the pencil and 

 sweeps to the right of it. 



