564 ANIMAL PHYSIOLOGY. 
holding up before such an eye any small object, as a pair of 
forceps, it may be readily observed that an inverted image of 
the object is formed on the back of the eye (fundus). If, how- 
ever, the lens be removed from such an eye, no image is formed. 
If the lens be itself held behind the object, an inverted image 
will be thrown upon a piece: of paper held at a suitable (its 
focal) distance. By substituting an ordinary biconvex lens, the 
same effect follows. It thus appears, then, that the lens is the 
essential part of the refracting media, though the aqueous and 
vitreous humors and the cornea are also focusing mechanisms, 
The surfaces of the refracting media may all be considered 
to be centered on one of the axes, which meets the retina above 
and to the inner side of the fovea centralis. We may for 
practical purposes reason from a diagrammatic eye, the re- 
fracting surfaces of which are (1) the anterior surface of the 
cornea, (2) the anterior surface of the lens, and (3) the posterior 
surface of the lens. The media may be reduced to (1) the lens 
substance and (2) the aqueous, or, as it has about the same 
refracting power, the vitreous humor. 
By the posterior principal focus is meant the point at which 
all rays that fall on the cornea parallel to the optic axis are 
focused. It is 14°647 mm. behind the posterior surface of the, 
lens, or 22°647 mm. behind the anterior surface of the cornea in 
Fia. 408.—Refraction by convex lenses (after Flint and Weinhold). The lens may be assumed 
to consist of a series of lenses (II in figure), for the sake of simplicity, though of course 
this is not strictly accurate. 
the diagrammatic eye. In the actual eye the fovea of the retina 
must occupy this position when at rest, if a distinct image 18 
to be formed. 
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