542 



THE NERVOUS SYSTEM. 



FIG. 140. 



the cornea, b. Upright 



c. Inverted image, from 

 the posterior surface of 

 the lens. (Helmholtz.) 



and if the observer place himself on the opposite side, at an equal angle 

 with the line of sight, three reflected images of the flame will become 

 visible, as in Fig. 140. 



The first image (a), which is the brightest, and upright, is reflected 

 from the cornea. The second (b), which is also upright, but much 

 fainter, is from the convex anterior surface of the lens ; and the third 

 (c), which is tolerably distinct, but inverted, is 

 from the posterior surface of the lens, acting as a 

 concave mirror. If the person under observation 

 now change his point of sight, from a distant to a 

 near object, the eyeball remaining fixed, the second 

 image (b) becomes smaller, and places itself nearer 

 the first. This indicates that the anterior surface 

 of the lens becomes more prominent, and approaches 

 a * c the cornea ; but there is no change in the other two 



CATOPTRIC IMAGES IN images, showing that the curvatures of the cornea 

 and posterior surface of the lens remain unaltered. 

 Helmholtz has made these phenomena more ap- 

 parent by employing, instead of a single light, two 

 sources of illumination in the same vertical line 

 (Fig. 141). This gives two catoptric images, one 

 above the other, for each surface of reflection ; 

 and a change in convexity of either one would be manifested by the 

 approach or separation of its images. In accommodation for remote 

 objects (^4), the images from the anterior surface of the lens are rather 



large and widely separated; in 



FIG. 141. accommodation for near objects 



(B), they diminish in size and 

 approach each other. The reflec- 

 tions from the cornea and those 

 from the posterior surface of the 

 lens remain at the same distance 

 in both states of accommodation. 



The advance of the iris and 

 pupil, from protrusion of the an- 

 terior face of the lens, in accom- 

 modation for near objects, can be 

 observed, as remarked by Helm- 

 holtz, by looking into the eye from 

 the side. The person under ob- 

 servation fixes his sight upon a 



distant object, and the observer places himself in such a position that 

 the edge of the iris is just concealed by the sclerotic. If the sight be 

 now shifted from the distant object to a nearer one in the same linear 

 range, the pupil visibly advances toward the cornea, and the iris shows 

 itself a little in front of its former position. If the sight be again 



CHANGE OP POSITION IN DOUBLE CATOPTRIC 

 IMAGES during accommodation. A. Position 

 of the images in accommodation for distant 

 objects. B. Position of the images in accom- 

 modation for near objects, a. Corneal image. 

 b. Image from anterior surface of lens. c. 

 Image from posterior surface of lens. (Helm- 

 holtz.) 



