238 



GENERAL SCIENCE 



FIG. 213. Two Cameras. 

 A diagram showing the similarity in 

 the structure of a photographic camera 

 and the eye. 



their shapes so that objects at different distances may 

 be properly focused. The crystalline lens is elastic and 

 would become more convex if it were not held under 



tension by the suspen- 

 sory ligament which is 

 attached to the choroid 

 coat. When the distance 

 of the object requires a 

 change of focus, the cil- 

 iary muscles draw the 

 choroid coat forward, 

 thus reducing the tension 

 on the crystalline lens. 

 The lens immediately be- 

 comes more convex and 

 shortens its focus. When the ciliary muscles are relaxed 

 the crystalline lens is pulled back to its original position. 

 The shutter (iris) in the eye works automatically, al- 

 lowing the proper amount of light to enter the eye at all 

 times. The retina is also superior to the camera. While 

 the photographic plate can be 

 used but once, the retina is able 

 to receive any number of impres- 

 sions and send them to the brain 

 for record. In other words, the 

 camera of the eye is always 

 " loaded." 



The image formed on the ret- 

 ina is always inverted (Figure 

 214), but this does not prevent our seeing the object in 

 its correct position ; since it is the brain which translates 

 the visual impulse of the retina into the sensation of 

 actual sight, 



FIG. 214. Showing the 

 Formation of an Image upon 

 the Retina. 



