I08 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



bination is achromatic. Now the eye also is corrected 

 for chromatism, otherwise all objects would appear 

 fringed with colors. The mode of correction also is the 

 same, for the eye also consists of several lenses differing 

 in curvature and in material. In fact, the structure of 

 the eye gave the first hint as to the proper mode of 

 correcting lenses — i. e., by combination. 



2. Aberration. — In ordinary lenses (i. e., those 

 with spherical curvature) it is found that the marginal 

 rays are refracted too much for the central rays, and 

 therefore all the rays are not brought together to the 

 same focus. This may be partly remedied by cutting 

 off the marginal rays by a diaphragm, but, of course, 

 with great loss of light. But it can be completely reme- 

 died only by making the central part of 

 the lens more refractive. This can be done 

 either by graduating the density of the 

 matter of the lens from the margin to the 

 center, or else by graduating the curvature 

 from margin to center. The first method 



'tioi/siiowing art nas found impossible to accomplish, 

 the structure an( j therefore it adopts the second method. 



of the lens. 



Instead of a spherical curvature, it makes 

 an elliptical curvature, the axis of the ellipse being the 

 axis of the lens. In this way the best lenses are cor- 

 rected for aberration. 



Now the eye also is corrected, for otherwise it could 

 not sharply define the objects it looks at. How is it cor- 

 rected ? It is probable that it uses both methods. The crys- 

 talline lens consists of concentric layers, becoming denser 

 and denser to the center (Fig. 76). Also the curvature 

 of the corneal surface is elliptical instead of spherical. 



3. Adjustment for Distance. Focal Adjustment. 

 Accommodation. — We have seen that in order to have a 

 good image the receiving screen must be at the exactly 



