ORGANS OF SPECIAL SENSE THE EYE 



349 



the retina, as if the brain were looking at the image on the 

 retina. The exact process by which the brain perceives the 

 image is complicated, and not entirely known. For our pur- 

 pose it is sufficient to say that 

 although the image is certainly 

 inverted on the retina, after the 

 impression has reached the brain, 

 it is interpreted so that we see 

 things as they are. 



Accommodation. Every one 

 knows that it is perfectly pos- 

 sible to look at a spot on the 

 window glass and see it clearly, 

 and at the same time see indis- 

 tinctly whatever there may be beyond the glass, trees, build- 

 ings etc. If one chooses he can give all his attention to the 

 trees and see them very clearly, at the same time seeing very 



FIG. 172. DIAGRAM 



Showing the formation of an im- 

 age upon the retina. 



FIG. 173. DIAGRAM 



Showing the effect of different lenses in changing the position of the focus of light' 

 and illustrating how a change of the lens is necessary if objects at different dis- 

 tances are to be focused upon a screen. 



indistinctly the window glass and frame. When he changes 

 his attention from one to the other he is conscious that some- 

 thing is taking place in his eyes, and that it requires part of 



