462 THE SPECIAIi SENSES 



scattered rays of a luminous body that a sharp but in- 

 verted image of the object is thrown upon any screen 

 placed at the point of convergence. (See Fig. 219.) In 



Fig. 219 — Convex lens of eye forming an image by converging the rays on tlie 



retina. 



a camera a glass lens serves this purpose. In the eye, the 

 cornea and crystalline lens accomplish the same result. 



Accommodation. — In a camera with a lens of a definite 

 shape the images formed by the converging rays are 

 focussed at different distances back of the lens according 

 as the objects which produce the rays are near to or far 

 from the lens. To secure a sharp image for any given 

 object such instruments are provided with mechanical 

 adjustments by means of which either the lens or the 

 screen may be moved until the distance between them is 

 such that the screen is exactly at the point of convergence 

 of the rays. Such an adjustment is called accommodation. 

 This accommodation to distant and near objects may also 

 be secured by varying the thickness of the lens, and in the 

 eye it is this latter method that is employed. As has 

 been seen, the cornea and retina ("screen) are fixed. By 

 means of the ciliary muscles and suspensory ligament, 



