THE SENSE ORGANS 421 



retina as you would focus a picture on the film of your camera. 

 The iris controls the amount of light entering the eye and the lens 

 muscles change its shape so that the picture on the retina may be 

 sharp and clear. The retina is affected by the light that falls 

 upon it and the impression is carried to the brain by the optic 

 nerve, as sight. 



Protection of the Eye. Obviously, the eye cannot be buried in 

 the skull for protection, like the ear, but it is well guarded none the 

 less. The bony socket, walled in by the forehead, nose and cheek 

 ward off any but direct blows. The pad of fat on which it rests 

 saves it from jar or pressure. The eyebrows keep out perspiration 

 and the lids and lashes protect from dust. Tear secretion con- 

 stantly washes the front surface and a complicated set of reflex 

 actions helps us to ward off most injuries to this important sense 

 organ. 



The Living Camera. The eye is often compared to a camera 

 and there are so many resemblances, that it may be helpful to 

 study this table of comparisons. 



Part of eye corresponding to Part of Camera 



Ball Camera box 



Lens Lens 



Lids Shutter 



Iris Stops or diaphragm 



Pupil Lens opening 



Lens muscles Focusing devices 



Black pigment Black lining 



Retina Plate or film 



In making this comparison it must always be borne in mind that 

 there are also fundamental differences. The eye is alive, the camera 

 is not. The eye produces a sensation which reaches the brain, the 

 camera makes a picture. The eye focuses by changing the 

 shape of the lens, the camera, by changing its distance from the 

 film. 



