THE SENSES in 



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 ~^^_^^ J 



nearer object will be focussed be- FlG> 47 ._T show that rays 

 hind the retina, and a clear image from distant and near objects 

 will not be formed (Fig. 4V). This ^l the reti 

 means that near and far objects 



cannot be distinctly seen at the same time, a fact which 

 can be readily demonstrated by Scheiners Experiment. 

 (Experiment.) 



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

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

 in front of the other. 

 Get some one to 

 hold a needle against 

 a white sheet of 

 paper at about three 



x L .. ,, FIG. 48. Schemer s Experiment. 



yards from the eye, 



and hold another needle in the same line at about a foot 

 from the eye. When the far needle is looked at the near 

 needle becomes double. 



It is found practically that objects at a greater distance 

 than 6 metres may 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 vision 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 

 vision), becomes greater and greater till it can increase no 

 further when the near point is reached. The change is 

 called positive accommodation, and it consists in an in- 



