498 ACCOMMODATION. [Boon m. 



which can be ascertained to take place in the eye during accommo- 

 dation for near objects. 



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

 the pupil becomes small; when wo 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. 



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: one a very bright 

 one caused by the anterior surface of the cornea, a second less 

 bright, by the anterior surface of the lens, and a third very dim, 

 by the posterior surface of the lens ; 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 anterior surface 

 of the lens, is seen to become distinctly smaller, shewing 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 diminished, 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 shewn 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 accommodation 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. When the 

 anterior surface of the lens becomes more convex the two images 

 reflected from that surface approach each other, when it becomes less 

 convex they retire from each other. 



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 shewn by the fact that after removal of 

 the lens, as in the operation for cataract, the power of accommo- 



