EYES AND CAMERAS. 109 



may have lenses and even holes in them (though he has yet to 

 see the hole), they cannot be good cameras unless they have a 

 shutter to keep out light when not in use, a diaphragm before 

 the lens to exclude the light when it is too strong, a movable 

 lens so that they may be focussed, and a sensitive plate or 

 film for the picture to fall upon. "And eyes," says the man 

 who owns the camera, " have no shutters, nor diaphragms, nor 

 focus-screws, nor sensitive films." 



But our eyelids are shutters ; whenever we open our eyes 

 we " make an exposure," and we take pictures till we close 

 the lids over them. The eye is the first and best of cameras, 

 more wonderful than any other. That colored ring, the iris, 

 which sometimes is narrow and sometimes wide, is the device 

 which lets in more or less light as it is required. It adjusts 

 itself without thought on our part. The black spot in the 

 centre, the pupil, is the hole we spoke of. It is covered by a 

 transparent plate, the cornea, which keeps out the dust, but 

 through which we can look just as we do through a window. 

 The blackness of the pupil is merely a portion of the black 

 lining of the inner surface of the eyeball which we see through 

 the opening in the iris. Watch the eye of your cat, your dog, 

 or any other creature, and observe the way in which the iris 

 expands and contracts, and notice the varying shape of the 

 pupil. How does the pupil of a horse^s or a cow's eye differ 

 from a cat's, and how do both differ from a dog's ? 



But if an eye is a camera, how is it focussed ? Not by 

 turning a screw to regulate the distance of the lens from the 

 object j)ictured, but by changing the shape of the lens itself. 

 We need not think about this change at all, for it acts of itself, 

 or automatically, as we say. It will be long before any camera 

 is advertised with an automatic focus. As to sensitive films, 

 our eyes are furnished with the best to be had, one that does 



