236 THE WONDERFUL HOUSE THAT JACK HAS 



stand how we see. The pupil of the eye serves the 

 same purpose as the little hole in the camera which 

 is uncovered to let in the light while a picture is being 

 taken. The crystalline lens of the eye corresponds 

 to the glass lens of the camera, and the retina to the 

 sensitive plate or film on which the impression or 

 picture is taken. This sensitive plate or film is taken 

 from the camera, and the picture developed or brought 

 out. But in our eyes, the optic nerve transmits each 

 impression received on the retina to the brain, so 

 the same sensitive plate, the retina, is used all the 

 time. 



Perhaps we know that after the sensitive plate, or 

 film, has been taken out of a camera, and developed 

 so that the picture is visible, it is called a negative. 

 We may also be familiar with the fact that a negative 

 will not be clear unless the rays of light entering the 

 camera are brought to a focus exactly as they reach the 

 sensitive plate or film. The same thing is true of the 

 eye. Vision is not clear and distinct unless the rays of 

 light are brought to a focus exactly on reaching the 

 retina. If they are focused before or after reaching 

 the retina, sight is more or less blurred. 



When we are gazing at a distant object, if our eyes 

 are neither nearsighted nor farsighted, the rays of 

 light entering the eye are focused on the retina as 

 seen in the cut on page 237 without effort on our 

 part ; but if we look at near objects, as in reading and 

 writing, an effort of the eye is necessary to focus the 



