PHYSIOLOGY FOR BEGINNERS 



left-hand hole, the right-hand image (Fig. 99, A). Since an 

 image seen on the left-hand side is one formed on the right- 

 hand half of the retina, closing the right-hand hole has in 

 this experiment cut off the image on the same side, the right- 

 hand side, of the retina. This shows that the rays from the 

 near needle coming through one hole have not crossed, or 

 met, or been brought to a focus with those coming through 

 the other hole before they struck the retina. That is, the 

 crystalline lens, when accommodated for the distant needle, is 

 not strong enough to bring the rays from the near needle to 

 a focus on the retina. If, on the other hand, you fix the eye 

 on the near needle, closing the right-hand hole will cut off the 



FIG. ioo. The changes in the lens in accommodation. 



A, adjusted for distant ; B, for near objects. 



c, cornea ; con, conjunctiva ; set, sclerotic ; ck, choroid ; c.p, ciliary process ; 

 c.m, ciliary muscle ; s.I, suspensory ligament. 



right-hand image of the distant needle (Fig. 99, B). But 

 the image seen on the right-hand is the one on the left-hand 

 side of the retina, so that closing one hole in this case cuts 

 off the image on the opposite side of the retina. This shows 

 that the rays from the distant needle coming through the two 

 holes have crossed each other, and were brought to a focus 

 before they reached the retina. That is to say, the crystalline 

 lens, when accommodated for the near needle, is too strong to 

 bring the rays from the distant needle to a focus on the 

 retina, but brings them to a focus too soon. 



How does the lens change its curvature ? The crystal- 

 line lens is elastic, that is, if its surface be made flatter by 



