750 SIGHT. 



One is that the pupil contracts. When we look at near objects, the 

 pupil becomes small ; when we turn to distant objects, it dilates. This, 

 however, cannot have more than an indirect influence on the formation of 

 the image ; the chief use of the contraction of the pupil in accommodation for 

 near objects is to cut off the more divergent circumferential rays of light. 



625. The other and really efficient change is that the anterior surface of 

 the lens becomes more convex. If a light be held before the eye, three reflected 

 images may, with care and under proper precautions, be seen by a bystander : 



a b c a b c a b c 



Diagram of Images reflected from the Eye.] 



one a very bright one caused by the anterior surface of the cornea (a), a 

 second less bright, by the anterior surface of the lens (b), and a third very 

 dim, by the posterior surface of the lens (c) ; when the images are those of 

 an* object, such as a candle, in which a top and bottom can be recognized, 

 the two former images are seen to be erect, but the third inverted. When 

 the eye is accommodated for near objects, no change is observed in either 

 the first or the third of these images ; but the second, that from the ante- 

 rior surface of the lens, is seen to become distinctly smaller, showing that 

 the surface has become more convex. When, on the contrary, vision is 

 directed from near to far objects, the image from the anterior surface of the 

 lens grows larger, indicating that the convexity of the surface has dimin- 

 ished, while no change takes place in the curvature either of the cornea or 

 of the posterior surface of the lens. And accurate measurements of the 

 size of the image from the anterior surface of the lens have shown that the 

 variations in curvature which do take place are sufficient to account for the 

 power of accommodation which the eye possesses. 



The observation of these reflected images is facilitated by the simple instrument 

 introduced by Helmholtz and called a phakoscope. It consists of a small, dark 

 chamber, with apertures for the observed and observing eyes ; a needle is fixed at 

 a short distance in front of the former, to serve as a near object, for which accom- 

 modation has to be made ; and a lamp or candle is so disposed as to throw an image 

 on each of the three surfaces of the observed eye. Since the distance between two 

 images is more readily appreciated than is a simple change of size of a single image, 

 two prisms are employed so as to throw a double image of the lamp on each of the 

 three surfaces [Fig. 169, B. C ]. When the anterior surface of the lens becomes 

 more convex the two images reflected from that surface approach each other (C ), 

 when it becomes less convex they retire from each other (B). 



These observations leave no doubt that the essential change by which 

 accommodation is effected is an alteration of the convexity of the anterior 

 surface of the lens. And that the lens is the agent of accommodation is 

 further shown by the fact that after removal of the lens, as in the operation 

 for cataract, the power of accommodation is lost. In the cases which have 

 been recorded, where eyes from which the lens had been removed seemed 



