Fig. 1

To prove the existence of the blind spot close your left eye and direct the right eye to the small cross on the left hand side of the figure. Hold your face ten or twelve inches away from the screen, and then gradually bring it nearer, still keeping the gaze fixed upon the cross. The round spot will also be visible, except at a certain distance from the eye - about seven to ten inches - when it will disappear from view. Its image falls upon the point of entrance of the optic nerve, which is incapable of perceiving light.


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