800 PHYSIOLOGY. 



BLIND SroT. Marriotte's experiment proves that at the en- 

 trance of the optic nerve, where rods and cones are not to be found, 

 the spot is a blind spot. 



Thus: make a cross and a circle about three inches apart upon 

 paper. With the right eye view the cross, keeping the left eye 



A B 



Fig. 348. Purkinje-Sanson Images. (BALL.) 



A, In the absence of accommodation. B, In accommodation. 1, Reflection 

 from the cornea. 2, From the anterior surface of the lens. 3, From the poste- 

 rior surface of the lens. 



closed. Hold the paper about a foot from the eye, when both cross 

 and circle will be visible. Let the paper be gradually brought nearer 

 the eye, keeping the right eye steadily fixed on the cross. At a cer- 

 tain moment the circle will disappear, and that time is when the 

 image of the circle falls upon the optic nerve entrance. 



Fig. 349. Diagram to Show the Blind Spot in the Visual Field. 



( BALL. ) 



The minimum visual angle is fifty seconds; below this limit the 

 extreme points of an object are no longer separate and but one point 

 is perceived. The minimum visual angle corresponds to a retinal 

 image of about .004 millimeter, which is nearly the diameter of one 

 of the cones of the retina. 



ACUTENESS OF VISION. Acuteness of vision is in inverse ratio 

 to the visual angle. It diminishes as the later increases. In favor- 

 able conditions, bodies having a diameter of 1 / 40 to Vioo of an inch 

 are perceived by the naked eye. 



