146 VETERINARY PHYSIOLOGY 



the cornea and aqueous, and a biconvex part, the crystalline 

 lens, behind. In the resting normal eye (emmetropic eye) the 

 principal focus is exactly the distance behind the lens at 

 which the layer of rods and cones in the retina is situated, 

 and thus it is upon these that light, coming from luminous 

 points at a distance, is focussed. 



2. Near Vision— Positive Accommodation. — If an object is 

 brought nearer and nearer to the eye, the rays of light enter- 

 ing the eye become more and more 

 divergent, and if the eye be set so 

 '.-.---■■'- that rays from a distance — i.e. 

 parallel rays — are focussed, then 

 Fig. 66. -To ^w that rays ^'ays from a nearer object will be 

 from distant and near focussed behind the retina, and a 



°h*«"irat:he'rrl°e° clear image will not be formed 



(fig. 6). This means that near 

 and far objects cannot be distinctly seen at the same time, 

 a fact which can be readily demonstrated by Scheiner's 

 Experiment {Practical Physiology). 



Make two pinholes in a card so near that they fall 

 within the diameter of the pupil. Close one eye and place 

 the holes in front of the other. Get someone to hold a 

 needle against a sheet of white paper at about three 



Fig. 67.— Scheiner's Experiment represents rays from the near needle 



when it is looked at, and - - - - rays from the far needle. 



yards from the eye, and hold another needle in the same 

 line at about a foot from the eye. When the near needle 

 is looked at, the far needle becomes double (fig. 67). 



In man it is found that objects at a greater distance than 

 6 metres may practically be considered as " distant," and that 

 they are focussed on the retina. 



Objects may be brought nearer and nearer to the eye, and 

 yet be seen distinctly up to a certain point, the near point of 

 accommodation within which they cannot be sharply focussed 



