THE EYE AS AN OPTICAL INSTRUMENT. 



209 



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

 do the bli;od vessels of the retina enter the fovea^ but end 

 in a circle of delicate capillaries around it. 



^ %Si 



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

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



