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VETERINARY PHYSIOLOGY 



pound lens composed of a convexo-concave part in front, the 

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

 behind. In the resting normal eye (emmetropie 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. 



Positive Accommodation. If an object is brought nearer and 



nearer to the eye, the rays of light 

 entering 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 

 rays from a nearer object will be 

 focussed behind the retina, and a 

 clear image will not be formed (fig. 

 55). This means that near and far objects cannot be distinctly 

 seen at the same time, a fact which can be readily demon- 

 strated by Schemer's Experiment. (Practical Physiology.) 



Make two pin-holes in a card so near that they fall within 

 the diameter of the pupil. Close one eye, and hold the holes 

 in front of the other. Get someone to hold a needle against a 

 sheet of white paper at about three 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. 56). 



FlG. 5f>. To show that rays 

 from distant and near objects 

 are not focussed on the retina 

 at the same time. 



FIG. 56. Schemer's Experiment 



represents rays from the near needle 



and - - - - rays from the far needle. 



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

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

 aeeommodation within which they cannot be sharply focussed 

 upon the retina. This, however, requires a change in the lens 

 arrangement of the eye, and this change, beginning when the 

 object comes within about 6 metres (the far point of aeeom- 



