280 GENERAL SCIENCE 



the size of the pupil to regulate the amount of light enter- 

 ing the eye. 



While we are in the sunlight or in a room where the 

 lights are bright, the pupil is small, but when we are in 

 the dark the pupil is very large. The changing of the 

 size of the pupils of your eyes can be seen by going sud- 

 denly from a dark room into a lighted room and watching 

 for the change by looking into a mirror; or by closing 

 the eyes for a moment in a lighted room and then open- 

 ing them suddenly and looking into a mirror. When we 

 go from the bright sunlight into a house, especially if 

 snow is on the ground, we cannot see distinctly for a 

 while, because the iris does not adjust itself quickly after 

 being in strong light for some time. 



Just back of the iris is the colorless crystalline lens, 

 which is double convex and refracts the light and forms 

 an image just as does the lens in a magnifying glass. 

 The crystalline lens is held in position by a ligament 

 which goes entirely around it, much like the frame of 

 a magnifying glass which holds the lens. The crystalline 

 lens can change its shape and can become thicker or 

 thinner through the center. When the ligament around 

 the lens is loosened by the action of a muscle, the thick- 

 ness of the lens is increased. When this circular muscle 

 relaxes, then the ligament in which the lens is held pulls 

 outward and makes the lens thin but broader. When 

 we read, the lens is made thick; when we observe distant 

 objects the lens is made thin or less convex, so that the 

 image formed by it falls on the right place. 



Back of the crystalline lens is the largest part of the 

 eye, which is filled with a transparent substance called 

 vitreous humor. This helps to keep the eyeball in shape 

 and acts somewhat as a lens. The retina lines the entire 



