54 EVOLUTION OF THE HUMAN EYE 



The high acuity of form-sense at the fovea may be due 

 to: 



(a) The close congregation of cones together in that 

 region. 



(b) The isolation of each cone in that region to one nerve 

 path. 



(c) The opening out of the inner layers of the retina, 

 allowing the unimpeded access of rays of light to the cones. 



Probably all three factors are essential for clearness of 

 definition in vision. 



If, as suggested in the consideration of the light-sense, 

 the response of the cones to light is a mechanical one, a 

 contraction produced in one of them being capable of start- 

 ing a stimulus in the isolated nerve path with which it is 

 connected, we have a very simple means of explaining the 

 first stage in the production of the perception of form. 



At the center of the fovea in the human retina we have 

 an area in which 7000 cones are supposed to be crowded 

 together; 50 cones may be counted along a line of 200 /z 

 in length; the average diameter of each cone at its widest 

 part is about 3 M, and the distance between two adjoining 

 cones 4 /z. Supposing now the image of a black letter on 

 a white ground, such as we have in our test-types, to be 

 focussed on this delicate mosaic of cones, those upon which 

 the white background is projected will be stimulated and 

 contract, those upon which the black letter fell, not being 

 stimulated, will remain uncontracted. Indeed, it seems 

 likely that if we could examine the outer surface of a retina 

 so stimulated, under sufficient magnification, a reproduction 

 of the letter looked at w r ould be seen raised up and composed 

 of the unretracted outer limbs of the cones. 



An abnormally low acuity of central form-sense dating 

 from birth, and unaccompanied by any obvious ophthal- 



