338 PRACTICAL PHYSIOLOGY. [LXXII. 



closed. Look steadily at the cross with the right eye, when both the 

 cross and the circle will be seen. Gradually approach the card 

 towards the eye, keeping the axis of vision fixed on the cross. At a 

 certain distance thecircle will disappear, i.e., when its image falls on 



FIG. 254. Marriotte's Experiment. 



the entrance of the optic nerve. On bringing the card nearer, the 

 circle reappears, the cross of course being visible all the time. 



(b. ) Perform the experiment in this way. Place the flat hand vertical to 

 the face, and with its edge touching the nose so as to form a septum between 

 the two fields of vision. Fix the cross in fig. 255, keep both eyes open, and 



FIG. 255. 



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

 disappear. 



(c. ) Close the left eye, and fix the point a (fig. 256) ; 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. 256 



(rf.) Volkmann's Experiment on the Blind Spot. 



Look at the spot a (fig. 257) with one eye, the gap, b c, disappears when it 

 falls on the blind spot and the line looks continuous ; the points b and c appear 

 as if placed in the same point of the field of vision, so that the parts of the 



