n6 HUMAN PHYSIOLOGY 



in front of the lesser curvature of the eye and thus equalises 

 it with the other curvature (Fig. 53). 



B. Stimulation of the Retina. 



1. Reaction to Varying Illuminations. (1) The Blind 

 Spot. At the entrance of the optic nerve the retina can- 

 not be stimulated because there are no end organs in that 

 situation. The existence of such a blind spot may be 



FIG. 54. Methods of demonstrating the Blind Spot, a, by Mariotte's 

 Experiment ; 6, by moving a pencil along a sheet of paper. 



demonstrated 1st, By Mariotte's experiment, which consists 

 in making two marks in a horizontal line on a piece of paper, 

 closing the left eye, fixing the right eye on the left-hand 

 mark with the paper held at a distance from the eye, when 

 both marks are visible, then bringing the paper nearer to the 

 eye, when the right-hand mark will first disappear, and when 

 the paper is brought still nearer will reappear (Fig. 54, a). 

 (Experiment) 2nd, By making a mark on a sheet of paper, 

 and with the head close to the paper moving the point of a 

 pencil to the right for the right eye, or to the left for the 

 left eye, when the point will disappear and again reappear 

 (Fig. 54, b). (Experiment.) 



