THE REFRACTING MEDIA OF THE EYE. 325 



front of the face, and moving it alternately from and towards 

 you. The eye must all the time be kept looking fixedly at 

 the cross. When the book is about ten inches from the 

 eye the white disc entirely disappears from view: its image 

 then falls on the part of the retina where the optic nerve 

 enters, and causes no visual sensation. 



Light consists of vibrations in an ether which pervades 

 space. An object which sets up no waves in the ether does 

 not excite the visual nervous apparatus, and appears black; 

 an object which sets up ethereal vibrations capable of 

 exciting the rods and cones of the retina appears white 

 or colored when we look at it. The ethereal vibrations 

 enter the eye through the cornea, pass on through the 

 pupil, and reach and stimulate the retina. 



The Refracting Media of the Eye are three in number: 

 (1) the aqueous "humor ; (2) the crystalline lens ; (3) the 

 vitteous humor. Their relative positions are shown in Fig. 

 88. These media act like a convex lens, such as a common 



FIG. 92. Illustrating: the formation behind a convex lens of a diminished 

 and inverted image of an object placed in front of it. 



burning-glass, and bend the rays of light which pass through 

 them (Fig. 92), so that all those which start from one point 

 of an external object meet again in a, focus on one point of 



What is light? When does an object appear black? 

 Name the refracting media of the eye. State their relative posi- 

 tion. Describe their action. 



