326 TB& HUMAN BODY. 



the retina. In this way a small and inverted image of the 

 things at which we look is formed on the retina, and stimu- 

 lates its rods and cones. 



Accommodation. In the healthy eyeball the crystalline 

 lens is controlled by muscles which change its convexity, 

 making this greater when we look at near objects, and less 

 when we look at distant objects. When the lens is very 



FIG. 93. Section of front part, of eyeball showing the change in the form 

 of the lens when near and distant objects are looked at. a, c, b, cornea; A, lens 

 when near object is looked at; /?, lens when distant object is looked at. 



convex we cannot see a distant object distinctly, and when 

 it is less convex we only dimly see a near object. For 

 example, standing at a window behind a lace curtain we 

 can look at the curtain and see its threads plainly, but while 

 so doing we only see indistinctly houses on the other side 

 of the street; because the convexity of the lens is then such 

 as to focus light from the near object on the retina, and not 

 that from the distant. We can, however, "look at" the 

 houses over the way and see them plainly; but then we no 

 longer see the curtain distinctly, because the lens has so 

 changed its form as to focus light from the far object on the 



When we look at an object, what is formed on Hie retina? 



How is the form of the crystalline lens controlled? When is its 

 convexity greater? Can we see near and distant objects distinctly 

 at the same moment? Illustrate. 



