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APPLIED PHYSIOLOGY 



Diagram of the formation of an image with a 



lens. 



a an object sending off light. 



b a lens which brings all the rays from any point 

 in the object together again into a single 

 point. 

 c image of the object a. 



a focus. If the focus falls upon a screen, an image of the object giving 



the light will appear. By changing the kind and the position of the 



lens the image may be 

 made either larger or 

 smaller than the real 

 object. 



Light has the power to 

 produce a chemical change 

 in substances. Photog- 

 raphy and bleaching 

 clothes are examples of 

 the action of light. In 

 photography a prepared 

 plate is inclosed in a tight 

 black box, into which light 

 from an object is admitted 

 through a small lens. 



The lens brings the light to a focus and forms an image upon the 



plate. 



605. The eye. The eye is an apparatus like a photog- 

 rapher's camera, but is more perfect. It consists of a 

 round, hollow shell about \ of an inch in diameter, formed 

 of a very tough membrane about -^ of an inch in thick- 

 ness, called the sclerotic coat. The sclerotic coat is lined 

 with a thin black membrane called the choroid coat, which 

 carries the blood tubes, and is colored black so as to 

 prevent reflection of the rays of light. Inside of the 

 choroid coat is a very thin and transparent membrane 

 called the retina. The cavity of the eyeball is filled 

 in front with a thin, clear liquid called the aqueous humor, 

 while its back part contains a thick, jellylike fluid called 

 the vitreous humor. The two humors keep the eyeball 

 distended and in shape. 



The nerve of sight, called the optic nerve, enters the 

 back part of the eye and separates into fine threads which 

 end in microscopic rods set closely together on their ends 



