272 EXPERIMENTAL PHYSIOLOGY. 



LESSON LXI. 



BLIND SPOT FOVEA CENTRAL! S DIRECT 



VISION MAXWELL'S EXPERIMENT - 



PHOSPHENES RETINAL SHADOWS. 



1. The Blind Spot. 



(a.) Marriotte's Experiment. On a white card make in the 

 same horizontal line a black cross and a circle about three 

 inches apart, as in Fig. 124. The cross and the circle may 



Fig. 124. Marriotte's Experiment. 



be black on white or coloured card. Hold the card verti- 

 cally about ten inches from the right eye, the left being 

 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 the circle will dis- 

 appear i.e., when its image falls on 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 



Fig. 125. 



nose so as to form a septum between the two fields of vision. 

 Fix the cross in Fig. 125, keep both eyes open, and on 



