278 ELEMENTARY PHYSIOLOGY. [XXVII. 



iocns befote they fall on the retina. Stop- 

 ping the right-hand hole removes the image 

 on the left-hand side of the retina ; and the 

 disappearance is referred to the right-hand 

 side. Construct a figure to shew this. 

 When a single image is seen, stopping either 

 hole merely makes the field of vision appear 

 dim. 

 This is called Scheiner's experiment. 



5. The image heing one, hring the needle closer to 

 the eye; at a certain distance it will become 

 double; this marks the near limit of accommoda- 

 tion. 



Fixing the needle on a sheet of paper, walk 

 away while looking at it through the two holes ; 

 at a certain distance it will become double, this 

 marks the far limit of accommodation. This 

 experiment succeeds best with short-sighted 

 people. 



Compare the near and far limits of accommoda- 

 tion as fixed by looking at a vertical needle 

 through horizontal holes with those fixed by 

 looking at a horizontal needle through holes 

 placed vertically. The results will differ ac- 

 cording to the amount of astigmatism in the 

 eye. 



6. Helmholtz's Fhakoscope. This should be 

 used in a dark room. The observed looks with 

 one eye through the hole opposite the needle. 

 The observer looks through the hole at the side. 



