240 ANIMAL PHYSIOLOGY. 



They are thus gathered together at a certain distance behind 

 the lens, in a focus situated in the direction of a line proceed- 

 ing through the centre of that body, from the visible point; 

 and the rays from every point being in like manner refracted, 

 an inverted image of the landscape is produced. 



Fig. 122. Diagram to illustrate the course of two cones of liglit to 

 be focused on the retina, and that distinct vision requires that 

 the focus for the object looked at correspond with the position 

 of the retina. 



But there are two sets of conditions required for the pro- 

 duction of a correct image on the retina : one is, that all the 

 rays from each spot, and their constituent colours, shall be 

 gathered by the crystalline lens and other refracting media 

 quite to a point; the other, that the position of the retina 

 shall correspond with the focus. 



As regards the first of these conditions, it will be remem- 

 bered, in the first place, that only parabolic surfaces have the 

 property of bringing all the rays from each point to a perfect 

 focus, and that an ordinary glass lens has an imperfection 

 dependent on its spherical curves, termed spherical aberration. 

 The crystalline lens has its surfaces approaching, probably, 

 pretty near to the parabolic form, but it must not be for- 

 gotten that it is not the sole refracting medium in the eye. 

 Thus, persons who have had the crystalline lens extracted 

 for the disease called cataract, have still an inverted image 

 thrown on the retina, and continue to see, although imper- 

 fectly, without the use of spectacles ; thus, also, the curvature 

 of the cornea, when too great, produces short-sightedness, by 

 making the refractive apparatus too powerful. It is, there- 

 fore, proper to note that it is a law of spherical aberration, 

 that, while it is exhibited very greatly in the rays which 

 pass through the lateral parts of a lens, it scarcely affects 



