ACCOMMODATION 



531 



FIG. 225. The reflected image from the 

 cornea (to the left), that from the an- 

 terior surface of the lens (middle pic- 

 ture), and that from the posterior sur- 

 face (to the right), after Helmholtz. 

 A, as seen in an eye adjusted for 

 distant vision; B, as seen in an eye 

 accommodated for near vision. 



it. Now it has been shown that in accommodation for near vision the image 

 reflected from the cornea does not change, while that from the anterior surface 

 of the lens, and to a less extent also that from the posterior surface, does. 

 That is to say, the two surfaces of the 

 lens become bulged out more in accom- 

 modation, the anterior however to a much 

 greater extent than the posterior. 



In order to observe these changes of 

 form, the eye to be examined is given two 

 definite points to look at (/ and n. Fig. 

 224), lying in the same straight line di- 

 rectly in front of it. Then two slits of 

 light from a large bright lamp flame sit- 

 uated to one side of the line of vision and 

 on the same level with the eye are thrown 

 into the eye. In Fig. 224 let A be the 

 observed eye and C the flame, B the eye 

 of the observer. If now the observer move 

 his eye back and forth in the neighbor- 

 hood of By so that the angle B A f is, approximately equal to C A f, he should 

 see three pairs of images reflected from the observed eye, namely, a (Fig. 225), 

 the brightest coming from the cornea, and b and c from the anterior and 

 posterior surfaces of the lens. When the eye is adjusted for distant vision 

 the images have the appearance of Fig. 225, A', for near vision (Fig. 

 225, 5), the image a does not change, but the image & becomes very much 

 smaller. By very exact methods it can be shown also that c becomes slightly 

 smaller. 



With these changes in the form of the lens the pressure in the anterior 

 chamber of the eye does not increase, neither does the posterior surface of 



the lens change its position, hence the 

 lens as a whole is not displaced. 

 But the anterior surface of the lens 

 presses forward and pushes the iris 

 before it. This can be seen by look- 

 ing at the cornea of the eye of an- 

 other person from the side and a lit- 

 tle to the rear (Fig. 226). With 

 the observed eye adjusted for distant 

 vision, the observer should place him- 

 self so that he can see just a little 

 of the black pupil projecting in front 

 of the sclerotic. Then when the eye 

 is accommodated for near vision, the 



pupil becomes much more plainly visible. We have already called attention 

 to the fact that the pupil itself becomes narrower in accommodation. 



According- to O. Weiss, the radius of curvature of the anterior surface of 

 the lens accommodated for a distance of 749 mm., is 9 mm. ; for 337 mm., 8 mm. ; 

 and for 199 mm., 7 mm. 



FIG. 226. The protrusion of the iris in accom- 

 modation, after Helmholtz. A, an eye ad- 

 justed for distant vision; B, an eye adjusted 

 for near vision. 



