THE SENSE OF SIGHT. 



659 



point reflected from the anterior surface of the cornea and 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 and two at the pupillary 

 margin (Fig. 319). 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 reflec- 

 ted from the anterior surface of the lens, the last from 

 the posterior surface. These images of reflection are 

 known as catoptric images. If now the individual be 

 directed to fix the gaze on a near object, the second im- 

 age 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 possible increase 

 in the convexity of the posterior surface of the lens. 



According to Helmholtz, during accommodation the 

 entire anterior surface of the lens becomes more convex, 

 while at the same time it slightly advances, possibly as 

 much as 0.4 mm. in extreme efforts. This change is represented in Fig. 

 320. According to Tscherning, the increase in convexity of the anterior 

 surface is confined to the central portion, the remainder of the surface 

 becoming somewhat flattened. There is, 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 



FIG. 319. CATOP- 

 TRIC IMAGES IN THE 

 EYE. a. Upright 

 image of reflection, 

 from the cornea, b. 

 Upright image from 

 the anterior surface 

 of the lens. c. In- 

 verted image, from 

 the posterior surface 

 of the lens. (Helm- 

 holtz.) 



FIG. 320. THE LEFT HALF REPRESENTS THE EYE IN A STATE OF REST. THE RIGHT 

 HALF IN STATE OF ACCOMMODATION. 



view, though of late Hess has brought forward definite experimental^evi- 

 dence in favor of the view of Helmholtz. The radius of curvature in either 

 case approximates 6 mm. in extreme efforts of accommodation. The in- 

 crease in convexity naturally increases 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 pronounced must be the 

 increase in convexity in order that they may be sufficiently converged and 



