860 A MANUAL OF PHYSIOLOGY 



Put a weak concave lens in front of the eye ; the image now falls 

 more nearly on the retina. Move the retina forward, so that the 

 focus is behind it. This corresponds to the hypermetropic eye. 

 Put a weak convex lens in front of the eye to correct the defect. 



(d] Observe that a plate with a hole in it, placed in front of the 

 eye, renders an indistinctly focussed image somewhat sharper by 

 cutting off the more divergent peripheral rays. 



(e) Fill with water the chamber in front of the curved glass that 



l- a;. 333. COMPOSITE PICTURE OF BLIND Srox (NOT REDUCED). 



The blind spot of the right eye was mapped by 31 men, the eye being always at a 

 distance of 12 inches from the paper. The maps were then superposed. The amount 

 of white at any point of the figure is intended to correspond to the number of maps 

 which overlapped at that point. Although the mechanical process of reproduction 

 gives rather an imperfect view of the composite map, the area in the centre of the figure 

 where the white is most continuous, and which represents the shape of the majority of 

 the blind spots, evidently bears a general resemblance to the outline in Fig. 332. 



represents the cornea. The focus is now behind the back of the eye 

 altogether. Refraction by the cornea is here abolished, as is the 

 case in vision under water. An additional lens inside the eye, or a 

 weaker one in front of it, corrects the defect. Fishes have a much 

 more nearly spherical lens than land animals, and a flat cornea. 



(/) Fill the hollow cylindrical lens with water, and place it in front 

 of the artificial eye. It is now astigmatic. A point of light is focussed 

 on the retina, not as a point, but as a line. The vertical and hori- 



