SPECIAL SENSES 373 



eye is like a camera. The cornea and lens of the eye cor- 

 respond to the lens of the camera, and the retina corresponds 

 to the sensitive plate. 



The pupil of the iris determines the amount of light that 

 shall enter the eye. 



How a lens forms an image. — ^A ray of light passing 

 from a rarer to a denser medium is bent or refracted from 

 a straight line. Therefore rays of light when they enter a 

 convex lens as shown in fig. 186, are bent so that they all 

 converge at point A behind the lens, that is, they come to a 

 focus at this point. Thus rays of light from the arrow 

 A — B passing 

 through the lens 

 form a picture on 

 the retina at h — a. 



Accommodation 

 of the eye to different 

 distances. — A photo- 

 grapher changes the 

 relative position of Fig- 186. Refraction of rays of light and for- 

 the camera lens and °^f °" °\ "■" ^^^^ °" *^ Jf}!'\ Yl°\" 



ject; b-a, image on retina. (After Brubaker.) 



the sensitive plate by 



focusing for near or far objects. He increases the distance 

 between the lens and the sensitive plate, which is at the 

 back of the camera, for near objects, and decreases it for 

 distant objects. When a camera is focused for a near ob- 

 ject it will give only a very indistinct picture of a far object. 

 And so with the eye. Hold up a pencil in front of the eye 

 between the eye and the door. Upon looking at the pencil 

 you see it clearly but see the door beyond indistinctly. Look- 

 ing beyond the pencil to the door, the door is seen clearly 

 and the pencil indistinctly. The eye in looking first at 

 a near and then at a far object changes its focus. The walls 

 of the eye, unlike the walls of the camera, are immovable, 

 but there is a very neat arrangement for changing the 



