SENSE OF SIGHT. 857 



distance behind the lens, a reversed image of the window will be formed 

 upon it. The manner in which this image is formed may be represented 

 in the following diagrams (Figs. 378 and 319). 



In these figures it is seen that rays from any point of the object, which 

 may be regarded as diverging rays, are brought to a point behind the 

 lens. If the figures should be completed, and lines drawn from each indi- 

 vidual point of the objects in the manner represented in the illustrations, it 

 must be evident in tracing each- of these lines that a small inverted image 



FIG. 378. DIAGRAM ILLUSTRATING THE FORMATION, BY A DOUBLE CONVEX 

 LENS, OF A SMALLER INVERTED IMAGE. (Oanot.) 



must be formed behind the lens. If a screen be placed at this point, 

 which corresponds to the focal length of the lens, it is evident that the 

 image will be distinctl}' defined. On the other hand, if the screen be 

 either approached or removed farther from the lens an indistinct image 

 will be formed. If the object be farther removed from the lens the 

 image will decrease in size, and to have a distinct image the screen must 

 be approached to the lens ; and, conversely, if the object be approached 



FIG. 379. DIAGRAM ILLUSTRATING THE FORMATION OF AN IMAGE BY A 

 DOUBLE CONVEX LENS. 



to the lens the image will be increased in size, and to have sharp definition 

 the screen must be moved farther from the lens. 



A similar process occurs within the eye, although the refraction of 

 rays of light is much more complicated than in the simple convex lens, 

 for in the eye the ray of light passes through several media and is 

 refracted by each. Nevertheless, in the eye, a small inverted image is 

 formed on the retina, as may be readily determined by removing the eye 

 from a recentty killed animal, and if the sclerotic be removed from the 



