REFRACTION IN THE EYE 485 



in the last medium are passing from the image 

 to the second (now the first) refracting surface. 

 Find the point at which these rays unite in the 

 first medium (now the second). 



The rays will unite at the original luminous 

 point. If the eye be accommodated or a light 

 placed in front of it, an image of the light will 

 be formed upon the retina. A portion of the 

 light rays entering the eye will be reflected from 

 this image. Passing back over their original course, 

 they will form in turn an image which will ex- 

 actly coincide with the luminous object. 



2. Draw a horizontal line as a visual axis. 

 Upon this visual axis draw two reduced eyes, 

 normal size (Fig. 65), facing each other a conven- 

 ient distance apart (5 cm.). Let the left be the 

 observer's eye, and the right the eye of the sub- 

 ject. On the visual axis behind the observer's 

 eye draw a lamp flame. Assume that the sub- 

 ject's eye is accommodated for this flame. 



The construction shows that were the observ- 

 er's eye away, an image of the flame would be 

 formed on the retina of the subject's eye. The 

 image would reflect light toward the flame. This 

 reflected light would enter by the observer's 

 eye, and the illuminated area of the subject's 

 retina thus be made visible, were it not that the 

 observer's eye is necessarily placed in the visual 



