DETERMINATION OF NEAR AND FAR POINTS. 267 



(6.) Close one eye, and with the other look through the 

 holes at the near needle, which will be seen distinctly, while 

 the far needle will be double, but both images are somewhat 

 dim (Fig. 121). 



(c.) With another card, while accommodating for the near 

 needle, close the right-hand hole, the right-hand image dis- 

 appears ; and if the left-hand hole be closed, the left-hand 

 image disappears. 



(d.) Accommodate for the far needle, the- near needle 

 appears double. Now close the right-hand hole, and the 

 left-hand image disappears ; and on closing the left-hand 

 hole, the right-hand image disappears (Fig. 121). 



(e.) Instead of using a card perforated with two holes, use 

 the apparatus provided for you, so constructed that one hole 

 is covered with a green and the other with a red glass. 

 Repeat the previous observations, noting the disappearance 

 of the red or green image, as the case may be. 



(f.) If desired, the holes in the card may be made one 

 above the other, but in this case the pin looked at must be 

 horizontal. 



Fig. 122. 



(g.) Make three holes in a piece of cardboard, as in Fig. 

 122, so that they can be brought simultaneously before one 

 eye, and look at a pin or needle. One sees three images of 

 the needle. On looking at a near object, the needles are in 

 the position b, and at a distant object in that shown in c. 



7. Determination of Near and Far Points. 



(a.) Place one of the vertical needles used in the previous 

 experiment conveniently, and with one eye the other eye 

 being closed look through the two holes in a card, and when 



