VISION 375 



i 



by the end of the optic nerve, and many other imperfec- 

 tions ; but it was shown by Purkinje many years ago that 

 by a very simple manoeuvre they may be forced upon our 

 notice. 



By making use of Purlcinje's figures, it can be proved that 

 the level in the retina at which undulations of light give rise 

 to the impulses which evoke visual sensations coincides with 

 the back of its anterior sheet i.e., with the layer of rods and 

 cones. A person stares fixedly at a white sheet in a dimly 

 lighted room while an assistant, by the help of a lens, focuses 



FIG. 29. PURKINJE'S SHADOWS. 



A beam of light traversing the eyeball in the direction A throws a shadow of the vessel v, 

 lying on the front of the retina, upon the sensitive layer at its back. When the light is 

 moved from A to B the shadow moves from a to 6. The mind, supposing the shadow to be 

 a dark mark on the nearest wall or screen, infers that this mark moves from A' to B'. 



a strong light on the front of his eyeball, to the outer side of 

 the cornea. The rays, traversing the white of the eye, throw 

 shadows of the retinal vessels on the layers behind them ; but 

 this not being the way in which light normally enters the eye- 

 ball, the person experimented upon supposes that he sees the 

 shadows in front of him. He mentally projects them on to 

 the white sheet. The pattern of his retinal vessels appears on 

 the sheet in grey streaks. When the spot of light is moved, the 

 shadow-pattern shifts, and in the same direction ; since, 



