THE SENSES. 



211 



the rays have not yet met, and at G they have already intersected each 

 other, and are again diverging. 



The retina must therefore be situated at the proper focal distance 

 from the lens, otherwise a defined image will not be formed; or, in other 

 words, the rays emitted by a given point of the object will not be col- 

 lected into a corresponding point of focus upon the retina. 



DEFECTS I:N T THE APPARATUS. 



A. Defects in the Refracting Media. Under this head we may 

 consider the defects known as (1) Myopia, (2) Hypermetropia, (3) Astig- 

 matism, (4) Spherical Aberration, (5) Chromatic Aberration. 



FIG. 380. Diagrams showing 1, normal (emmetropic) eye bringing parallel rays exactly to a 

 focus on the retiua; 2, normal eye adapted to a near-point; without accommodation the rays would 

 be focussed behind the retina, but by increasing the curvature of the anterior surface of the lens 

 (shown by a dotted line) the rays are focussed on the retina (as indicated by the meeting of the two 

 dotted lines); 3, hvpermetroptc eye; in this case the axis of the eye is shorter, and the lens flatter, 

 than normal; parallel rays are focussed behind the retina; 4, myopic eye; in this case the axis of the 

 eye is abnormally long, and the lens too convex; parallel rays are focussed in front of the retina. 



The normal (emmetropic) eye is so adjusted that parallel rays are 

 brought exactly to a focus on the retina without any effort of accommo- 

 dation (1, Fig. 380). Hence all objects except near ones (practically all 



