$6o A MANUAL OF VETERINARY PHYSIOLOGY 



much smaller than the object it represents ; it is a miniature 

 though perfect representation of the object presented to the 

 eye. The chief refraction undergone is at the anterior surface 

 of the cornea ; doubtless the other media also refract, the lens, 

 for example, but an eye can have very good distant vision 

 without a lens, the important function of which is tojprovide 

 the means for accommodation. Though the retinal picture is 

 so completely inverted that the right hand of the object becomes 

 the left of the image, and the top becomes the bottom, yet the 

 mind does not perceive the image as inverted, but mentally 

 refers the picture, not to the retina, but back to the object. In 

 Fig. 179, the angle XnY is equal to the angle YnX. The angle 

 XnY is spoken of as the Visual Angle, and all objects having 

 the same visual angle form the same sized picture on the retina. 

 By the aid of the visual angle the size of an image on the retina 



Fig. 179. — Diagram of the Formation of a Retinal 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 nodal 

 point n without being refracted ; the other rays, be and b'c, are refracted. 

 In this way the arrow XY forms a smaller inverted image of an arrow on the 

 retina YX. 



may be calculated, provided the distance of the nodal point 

 from the retina is known. Thus at the distance of a mile, a man 

 6 feet high is represented on the retina of a horse by an image 

 ¥ j Tr of an inch in height, in the human eye at the same distance 

 the picture of the man would be x^Vo" °f an inch, or about the size 

 of a red blood-corpuscle. The nearer the object the larger 

 the image ; taking the 6-foot man again at a distance of 10 yards,' 

 his height on the retina of the horse would be \ of an inch, whilst 

 on the retina of a man it would be rather over J of an inch. 



Theory of Vision. — The change that enables the vibratory 

 ether to start a nerve impulse by its action on the retina is 

 unknown. Cutting or stimulating the optic nerve causes no 

 pain, but produces a flash of light ; it is only when the physio- 

 logical stimulus occurs through the retina that an object is seen. 

 It is known that electrical changes occur in the eyes, associated 



