THE EYE AND THE PHYSIOLOGY OF VISION. 561 



shifting of the position of the retina, as in the telescope, 

 and surely the lens cannot move from the position in which 

 it is so firmly held by the suspensory ligament. The 

 change of focus is produced by a change in the curvature 

 of the lens. That this is true may be easily demonstrated 

 by the following experiment: If with suitable apparatus 

 one observe the images reflected out of the eye it is not at all 

 difficult to notice three such. A very clear reflection from 

 the front of the cornea, a second image reflected from the 

 front of the lens, and a third image from the posterior sur- 

 face of the lens. If these three surfaces remain in the same 

 position the relative positions of the images will remain, but 

 if one or more of these surfaces should change in any way, 

 the relative position and sizes of these images would vary. 

 If, now, a person from whose eyes such images are reflected 

 be asked to focus his eye for a distant object, and the posi- 

 tion of the images noted, and the person be then asked to 

 change the focus to a near object, two of the images change. 

 The image from the front of the cornea is unaffected, show- 

 ing that the cornea lias remained stationary. The image 

 from the back margin of the lens changes just a little, in 

 such a way as to show that this surface has become more 

 convex. The greatest change occurs to the image reflected 

 from the front of the lens. This moves much more rela- 



a, 6 # 



Fig. 176. THE REFLECTIONS OF TWO BRIGHT SQUARES FROM THE CORNEA, FRONT AND 



BACK OF LENS, A IN REST; B WHEN THE EYE IS ACCOMMODATED FOR A NEAR OB- 

 JECT. THE IMAGES a, a, FROM CORNEA DO NOT CHANGE, NOR DO THOSE c, c, FROM 



BACK OF LENS, BUT 6 IN FIGURE 23 IS MUCH SMALLER, SHOWING AN INCREASE IN 

 THE CONVEXITY OK THE FRONT OF LENS. 



lively and shows that the front of the lens has been pushed 

 forward and become much more rounded. A clearer proof 

 36 



