HOW THE EYE ADAPTS ITSELF 259 



into the inside of the eye, just as the diaphragm in the 

 camera controls the light. On a bright day, would you 

 expect the pupil of the eye to be larger or smaller than on 

 a dull day? Why? 



Experiment. To see the pupil of the eye change in size. 



From a room in which all the other windows are darkened look out of the 

 window into a very bright sky for a moment. Examine the eye in a 

 mirror. Then turn quickly and look at your eyes in the mirror with 

 your back to the light. Result? Explain. 



Control of focusing. The eye adjusts itself quickly 

 to see objects at different distances. This is done by 

 means of tiny muscles called the ciliary muscles, which 

 change the thickness of the lens. A flat lens focuses well 

 on distant objects, while a thick lens focuses on objects 

 near at hand. If you look at an object some distance 

 off, and then suddenly holding a book before your eyes try 

 to focus on the page, you will find that the print is blurred 

 for a moment before you are able to see. This change of 

 focus is called accommodation. 



How the eye adapts itself to different intensities of 

 light. It has been found that different intensities of 

 light in the home are desirable for different kinds of work. 

 The following table gives the intensity in foot candles, 

 which means the amount of light a candle would give at a 

 distance of one foot from an object. 



