SENSE OF SIGHT. 



633 



faces would be readily manifested by an approach or recession of the 

 two images belonging to it. In accommodation for remote objects (Fig. 

 200, A), the two images from the anterior surface of the lens are of 

 considerable size and somewhat widely separated ; in accommodation 

 for near objects (#), they diminish in size and approach each other. 

 The double reflections from the cornea and the posterior surface of the 

 lens, remain at sensibly the same 



distance from each other in both Fig. 200. 



states of accommodation. 



The advance of the iris and pu- 

 pil, in consequence of the protru- 

 sion of the anterior face of the lens, 

 as remarked by Helmholtz,can also 

 be observed directly, by looking 

 into the eye from the side. If the 

 observer look from this direction 

 so as to obtain a profile view of the 

 cornea and part of the sclerotic be- 

 tween the opening of the e3^elids, he 

 will see the dark pupil in perspec- 

 tive under the form of an upright 

 elongated oval, a little in front of 



the edge of the sclerotic. The person under observation fixes his sight 

 upon a distant object, and the observer places himself steadily in such 

 a position that the hither edge of the iris is just concealed by the ante- 

 rior border of the sclerotic. If the sight be now shifted from the dis- 

 tant to a near object, in the same linear range, the pupil visibly ad- 

 vances toward the cornea, and the edge of the iris shows itself a little 

 from behind the edge of the sclerotic. If the sight be again directed to 

 the distant object, the pupil recedes and the edge of the iris, disappears, 

 as before, behind the sclerotic. 



The accommodation of the eye for near objects is therefore produced 

 by an increased refractive power of the lens, from the greater bulging 



CHANGE OF POSITION IN DOUBLE 

 CATOPTRIC IMAGES during accommoda- 

 tion. A. Position of the images in accom- 

 modation for distant objects. B. Position 

 of the images in accommodation for near ob- 

 jects, a. Corneal image, b. Image from an- 

 terior surface of lens. c. Image from poste- 

 rior surface of lens. 



Fig. 201. 



Fig. 202. 



VISION FOR DISTANT OBJECTS. 



VISION FOR NEAR OBJECTS. 



of its anterior face. This has the effect of increasing the rapidity of 

 convergence of rays passing through it, and consequently compensates 

 for their greater divergence before entering its substance. In the ordi- 

 nary condition of ocular repose, when the eye is directed to distant ob- 

 41 



