THE EYE AS AN OPTICAL INSTEUMENT. 



209 



thinning away at its borders is seen in the diagram. Nor 

 do the blood vessels of the retina enter the fovea, but end 

 in a circle of delicate capillaries around it. 



This fovea, or pit of the retina, is of great importance 

 for vision, since it is the spot where the most exact dis- 



