lo6 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



the retina at a! b', and so of all intermediate points. If 

 the retina is properly placed the image will be perfect. 

 If the retina be too far back or, what is the same, if 

 the lenses are too refractive, the image will fall short of 

 the retina a' b' and will be blurred. This is the case in 

 the nearsighted. The object must be brought nearer, 

 so as to throw the image a little farther back. If the 

 retina be too near the lens or, which is the same, the 

 lenses too little refractive, the image will fall behind the 



Fig. 74. — Diagram illustrating the formation of an image on the retina. 



retina a" b" and will also be blurred. This is the case in 

 the old-sighted for near objects. The refraction must 

 be supplemented by glasses. These defects, however, 

 will be explained later. 



The fundamental fact that the images of all external 

 objects are really formed on the retina may be shown 

 in many ways. (1) Take the dead eye of an ox. Re- 

 move the coats on the back part of the ball, and replace 

 them by a mica plate — an inverted image of the land- 

 scape is seen. (2) The eyeball of a white rabbit shows 

 it without mutilation, because in these albinos the scle- 

 rotic is more transparent and the black pigment of the 

 choroid is wanting. (3) The image may be seen in the 

 living eye by the use of the ophthalmoscope. 



It is seen (Fig. 74) that the central rays of the pen- 

 cils cross one another at the nodal point. In the eye 

 the nodal point is a little behind the center of the lens. 



