Till. I.VK AND I HI M XSK < >K SKJHT 



lioles at the needles. When the eye is fixed on one needle, 

 one image of it is seen, and at the same time two images of 

 the other. One image of the needle on which the eye is 

 fixed is seen because the rays from it coming through the 

 two holes are brought to the same focus on the retina ; two 



Fie. 99. To illustrate accommodation. 



images of the other needle are seen because the rays coming 

 from it through the two holes are not brought to the same 

 focus but strike the retina in different places. Fix the eye 

 on the distant needle, and when you see the two images of 

 the near needle, with another card close one hole ; one of 

 the two images of the near needle disappears ; if the right- 

 hand hole be closed, the left-hand image disappears ; if the 



