110 EXPERIMENTAL PHYSIOLOGY 



box. Now move the lens back and forth until there ap- 

 pears on the screen a sharp image of the candle flame. Is 

 it right side up? What is the function of the lens? Mark 

 position of lens and screen. Move the candle three feet 

 farther away. Does the image remain on the screen? Keep- 

 ing the lens fixed, move the screen forward in the box until 

 the distinct image appears again. Return the screen to its 

 original position and, by moving the lens, cause the same 

 result — an image on the screen. Which is adjustable in 

 the eye — the screen (retina) or lens? How is the lens ad- 

 justed in the eye? (This adjustment of the lens to the dis- 

 tance of objects is called accommodation.) Change the disks 

 in the aperture, using first larger and then smaller openings. 

 Which gives the brightest image? What is the function of 

 the iris? 



Note. — By using external lenses as "spectacles, '' short- and long- 

 sightedness can be corrected and illustrated. 



