250 THE WONDERFUL HOUSE THAT JACK HAS 



Parallel Rays fo- 

 cused in front of 

 the Retina. 



are born with eyeballs so deep from front to rear 

 that the rays of light are brought to a focus before 

 reaching the retina. In such eyes, the 

 accommodation muscles are of little use. 

 In truth, they are even worse than 

 useless, because, when exercised, they 

 make the lens more convex, tending 

 to focus the rays of light sooner, thus 

 increasing, instead of lessening, the dif- 

 ficulty. Such a condition is called near- 

 sightedness. 



To make vision clear for nearsighted 

 people, it is necessary to prevent the 

 rays of light from being focused too 

 quickly after entering the pupil. This is done by 

 placing before the eyes glasses so hollowed out that 

 they are thinner at the center than at the edges. 

 Such glasses are called concave. By correcting the 

 defective form of the eyeball, they prevent rays of 

 light from being focused before reaching the retina, 

 thus making vision distinct. 



Nearsight is not always inherited, but unfortu- 

 nately is often caused by children's stooping over 

 books or holding them too near the eyes while reading 

 or studying. Such position interferes with the normal 

 circulation of blood in the eye, thus weakening the 

 tissues. As the latter are very soft and plastic in child- 

 hood, such long continued improper position gradually 

 changes the form of the eyeball, at the same time 



