lxxl] accommodation. 333 



7. Determination of Near and Far Points. 



(a.) Hold a pin vertically about ten inches in front of one eye, 

 the other eye being closed. Look through the two holes in the card 

 used for Scheiner's experiment, and when one distinct image of the 

 needle is seen, gradually approximate the needle to the cardboard ; 

 observe that it becomes double at a certain distance from the eye. 

 This indicates the near point of accommodation. 



(b.) Hold the card in front of one eye, and gradually walk back- 

 wards while looking at the needle, observing when it becomes 

 double. This indicates the far point of accommodation. N.B.^ 

 The experiment (6.) succeeds best in short-sighted individuals. 



(c.) Determine the near point with a vertical needle and card with hori- 

 zontal holes, and again with a horizontal needle and a card with the holes 

 vertical. The two measurements do not usually coincide, because the curva- 

 ture of the cornea is usually different in the two meridians. 



8. Purkinje-Sanson's Images. 



(a.) In a dark room Hght a candle, and hold it to one side of the. 

 observed eye and on a level with it. Ask the person to accommo- 

 date for a distant object, and look into his eye from the side 

 opposite to the candle, and three reflected images will be seen. At 

 the margin of the pupil, and superficially, one sees a small bright 

 erect image of the candle-flame reflected from the anterior svrface 

 of the cornea. In the middle of the pupil there is a second less 

 brilliant, larger, and not sharply defined ei-eet image. It is reflected 

 from the anterior surface of the lens. The third image, which lies 

 most posteriorly and towards the opposite margin of the pupil, is 

 the smallest of the three, and is an inverted image reflected from 

 the pof^ierior surface of the hns. Ask the person to accommodate 

 for a near object, and observe that the pupil contracts, while the 

 middle image— that from the anterior surface of the lens — becomes 

 smaller and comes nearer to the corneal image. This shows that 

 the anterior surface of the lens becomes more convex during 

 accommodation. 



{h.) Instead of using a candle-flame, cut two small square holes (lo mm. 

 square) in a piece of cardboard, and behind each place a gas-flame, and observe 

 the three pairs of square reflected images. 



(f.) Physical Experiment.— Place in a convenient position on a table a large 

 biconvex lens, supported on a stand. Standing in front of it, hold a watch- 

 glass in the left hand in front of the lens and a few inches from it. Move a 

 lighted candle at the side of this arrangement, and observe the three images 

 described above. Substitute a convex lens of shorter focus, and observe how 

 the images reflected from the lens become smaller. 



9. The Phakoscope of Helmholtz is used to demonstrate the 



