268 EXPERIMENTAL PHYSIOLOGY. 



one distinct image of the needle is seen, gradually approxi- 

 mate 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.) Holding the card in front of one eye, gradually walk 

 backwards while looking at the needle, observing when it 

 becomes double. This indicates the far point of accommo- 

 dation. N.B. The experiment (b.) succeeds best in short- 

 sighted individuals. 



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

 card with horizontal holes, and again with a horizontal 

 needle with 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, light a candle, and hold it to one side 

 of the observed eye and on a level with it. Ask the person 

 to accommodate 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 super- 

 ficially, one sees a small bright erect image of the candle 

 flame reflected from the anterior surface of the cornea. In 

 the middle of the pupil there is a second less brilliant and 

 not sharply defined erect image, which, of all the three 

 images, appears to lie most posteriorly. It is reflected from 

 the anterior surface of the lens. The third image lies towards 

 the opposite margin of the pupil, is the smallest of the three, 

 and is a sharp inverted image, from the posterior surface of 

 the lens. Ask the person to accommodate for a near object, 

 and observe that the pupil contracts, and 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 undergoes a change in 

 its curvature during accommodation. 



(b.) Instead of using a candle flame, cut two small square 

 holes (10 mm. square) in a piece of cardboard, and behind 

 each place a gas-flame, and observe the three pairs of square 

 reflected images. 



