LESSON XXIII. VISION. 



1. Retinal image. Remove very carefully the 

 sclerotic and choroid from a small portion of the posterior 

 surface of the eye of an ox or a sheep 1 . Place the eye in 

 the end of a blackened tube just large enough to hold it, 

 with the cornea outwards. 



Stand 6 to 10 yards from a window, and look towards 

 it ; an inverted image of the window will be seen on the 

 retina; move towards the window, the image will be- 

 come blurred. 



2. Accommodation. Standing some feet before a 

 window, close one eye and hold a needle up before the 

 other, at a distance of about six inches, so that it is at 

 right angles to one of the horizontal bars of the window. 



Look at the window-bar, the needle will appear dim 

 and diffuse. 



Look at the needle, the window-bar will appear dim 

 and diffuse. 



The eye can accommodate itself for either the needle or 

 the window-bar, but not for both at the same time. The 

 accommodation for the near object is accompanied by 

 a distinct feeling of effort. 



3. Diffusion circles and accommodation. Facing 

 a window, or a white surface, close one eye and hold a 

 fine needle vertically before the other. At about six 

 inches the needle will be seen distinctly. Bring it nearer 



1 Or better, take the eye of an albino rabbit, without cutting away 

 the sclerotic. 



