HOW IMAGES ARE MADE ON THE RETINA. 22Q 



at; and d, e, its image on the retina. The image is much 

 smaller than the object, and is wrong side up, but the 

 mind has learned by experience to understand it in the 

 right way. 



FIG. 56 Illustrating the formation behind a convex lens of a diminished and 

 inverted image of an object placed in front of it. 



8. Short-Sight and Long-Sight. When you use a tele- 

 scope or an opera-glass to look at any object, you have to 

 focus it. The arrangement which will enable you to 

 use it for seeing near objects distinctly, must be changed 

 before you can use the glass for seeing things farther 

 off. In our eyes, the lens does this focusing; it changes 

 according as we look at near or distant things. In per- 

 sons with good eyes (A, Fig. 57), the lens can accurately 

 focus on the retina, images of very distant objects, and 

 also of things within seven or eight inches of the eye. 

 In other persons (JB), the eyeball is too long from front 

 to back, and the lens cannot focus on the retina the 

 rays or lines of light coming from distant objects: such 

 persons are short-sighted. They can see very distinctly 

 things near the eye, but more distant objects seem 



of an object looked at, depicted on the retina? Why do we see it 

 rightly ? 



8. How is a telescope arranged for seeing near or distant objects ? 

 How do our eyes focus what they look at? What is said of this 

 power in good eyes ? Why are some eyes short-sighted ? Why are 

 others long-sighted? 



