VISION 115 



the eye, the image will be dim and diffuse, and at the 

 same time larger. The dimness and apparent increase 

 of size are due to diffusion, resulting from imperfect 

 accommodation. 



Prick a small hole in a piece of card, hold it before 

 the eye and again bring the needle close to the eye. It 

 will be seen distinctly at a much smaller distance than 

 before, and at the same time will appear magnified. It 

 will be seen more distinctly because the diffusion circles 

 are cut off. 



4. Limits of accommodation. Prick two small 

 holes in a card 1 to 1-5 mm. apart, place it touching the 

 nose and forehead, and look through the holes, say, with 

 the right eye. 



a. Look at the needle held about a foot away ; a single, 

 distinct image will be seen. The rays passing through 

 the two holes are united on the retina. 



b. Bring the needle closer to the eye; at a certain 

 distance it will become double; this marks the near 

 limit of accommodation. 



c. 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 accommodation as 

 fixed by looking at a vertical needle through horizontal 

 holes with those fixed by looking at a horizontal needle 

 through holes place vertically. The results will differ 

 according to the amount of astigmatism in the eye. 



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