6i6 



TEXT-BOOK OF PHYSIOLOGY. 



during accommodation, the necessary change, whatever it may be, 

 is to be sought for in the lens. As to the character of the changes 

 in this body, two views are held, based largely on the fact and its 

 interpretation, that images of a luminous point reflected from the 

 anterior surface of the cornea, the anterior and posterior surfaces 

 of the lens, change their relative positions during accommodation. 



Thus, if in a darkened room a lighted candle be placed in front 

 of and to the side of an individual whose eye is directed to a distant 

 object, an observer placed in the same relative position as the 

 candle will observe three images in the eye, one at the surface of the 

 cornea, two at the pupillary margin (Fig. 292). Of the two latter, 

 one is quite large and situated apparently in front of the third, which 

 is faint, small, and inverted. The middle image 

 is reflected from the convex surface of the lens, 

 thejast from the concave.surfaceffi oliese : images 

 of reflection are known as catofiric images. If 

 now the individual be directed to fix the gaze on a 

 near object, the second image changes its position, 

 advances toward the corneal image and at the 

 same time becomes smaller, a change which, in 

 accordance with the laws of optics, could only be 

 due to an increase in the convexity of the anterior 

 surface of the lens. A slight displacement of the 

 third image sometimes observed indicates a pos- 

 sible increase in the convexity of the posterior 



^/^S^}S^,9X^f^^ & 2>dt\.* frJfy dUrnO' d*A* tor* <** 

 According to Helmholtz, during accommoda- 

 tion the entire anterior surface of the lens becomes 

 more convex, while at the _same time it slightly 



7 *r~l^*1 I !!! "* O - 



advances, possibly as much as 0.4 mm. in extreme 

 efforts. This change is represented in Fig. 293. 

 According to Tscherning, the increase in convexity 

 * of the anterior surface is confined to the central portion, the re- 



It*- mainder of the surface becoming somewhat flattened. There is, 

 i ^moreover, no evidence that there is any advance of the surface or 

 any increase in the thickness of the lens. A series of new and 

 Ingenious experiments lend support to Tscherning' s view. The 

 radius of curvature in either case approximates 6 mm. in extreme 

 effort of accommodation. The increase in convexity naturally in- 

 creases the refracting power. 



Whichever view is accepted, the nearer the object, that is, the 

 greater the degree of divergence of the light rays, the more pro- 

 nounced must be the increase in convexity in order that they may be 

 ''^sufficiently converged and focalized on the retinal surface. Changes in 

 J .e convexity of the lens, either of increase or decrease, are attended 



i 

 a b c 



FIG. 292. CATOP- 

 TRIC IMAGES IN 

 THE EYE. a. 

 .-Upright image 

 of reflection, 

 from the cornea. 

 . Upright image 

 rom the ante- 

 'face~ or 

 s. c. In- 

 verted image, 

 from the poste- 

 rior^ surface of 

 the 1 e n s . 

 (Helmholfz.} 



