298 



A Manual of Veterinary Physiology. 



X n Y is spoken of as the visual angle, and it is equal to 

 the angle Y n X. All objects having the same visual angle 

 form the same sized picture on the retina, whether they be 

 near or far. 



The retinal image though inverted, is mentally referred 

 not to the retina, but to the direction from which it has 

 proceeded. Thus to the mind of the animal the picture is 

 not seen at Y A X, but referred to X ( > Y. 



The optical axis of the eye is the straight line through 

 the eye which passes through the centre of the refracting 

 media. 



The eye is liable to certain defects in its refracting media, 

 due either to its shape or errors in the curvature of the 

 refracting surfaces. 



Fig. 30. 



\.\i. Image | Foster). 



a, principal ray of the pencil of light proceeding from X ; a', principal 

 ray of the pencil of light proceeding from Y ; the principal rays 

 pass through the eye without being refracted ; the other rays b, o 

 and b', c' are refracted. In this way the arrow X O Y forms a 

 smaller inverted image of an arrow on the retina Y A X. 



Spherical Aberration.— The rays of light passing through 

 a convex lens are not all equally refracted, the rays passing 

 through the circumference being more bent than any 

 others; if the rays be derived from an object situated at 

 the side of the field of vision they do not all meet in the 

 same point, those passing through the circumference of the 

 lens coming to a focus earlier than those passing through 

 the centre. This defect, known as'spherical aberration,' 

 is remedied in the eye by the introduction of a diaphragm 

 or iris, which prevents some of the rays of light from 

 passing through the circumference of the lens. 



