340 



ELEMENTS OF PHYSIOLOGY 



right, in what direction does his image move ? The 

 reversals are explained by the crossing of the rays of 

 light as they pass through the lens. If two lenses of 

 different thickness be used, it will be found that the card- 

 board must be moved closer to catch the image from the 

 thicker lens. 



615. Accommodation (Fig. 216). Hold a pencil or finger 

 in line with some object, as a picture on the wall. When 



IRIS 



C.M. 



FlG. 216. Diagram to illustrate Accommodation. 

 Sc, sclerotic ; C.M . , ciliary muscle ; Sp. I, suspensory ligament ; C /, crystalline lens. 



looking at the finger, the picture is blurred and vice versa. 

 When looking up from a book that we are reading, to a 

 distant object, we do not realize that any change in the eye 

 is necessary ; but the lens changes 

 in shape, becoming more flattened 

 for the more distant object, and 

 becoming thicker again when a 

 near object is looked at, thus 

 always bringing the rays to a focus 



FIG. 217. 



Normal Eye, in which Parallel Rays , . 



of Light (A, A) focus the image upon the retina at whatever dis- 

 tance the object may be. But the 



power fails at a point called the near point, about four 

 inches from the eye for most persons, and the image 

 becomes indistinct. The change in shape of the lens is 



