MECHANISM OF REFRACTION IN THE EYE. 91 



and for the insertion of muscles. The iris has an action 

 similar to that of the diaphragm in optical instruments. The 

 suspensory ligament of the lens, the ciliary body, and the 

 ciliary muscle are for the fixation of the lens and its accom- 

 modation to distinct vision at different distances. The cho- 

 roid is a dark membrane for the absorption of light, prevent- 

 ing confusion of vision from reflection within the eye. 



Refraction by the cornea is effected simply by its external 

 surface. The rays of light from a distant point are deviated 

 by its convexity so that, if they were not again refracted by 

 the crystalline lens, they would be brought to a focus at a 

 point situated about fa of an inch behind the retina. 1 With- 

 out the crystalline lens, therefore, distinct vision is impossible, 

 though the sensation of light is appreciated. In cases of 

 extraction of the lens for cataract, the crystalline is supplied 

 by a convex lens placed before the eye. 



The rays of light, refracted by the anterior surface of 

 the cornea, are received upon the anterior surface of the crys- 

 talline lens, by which they are still farther refracted. Passing 

 through the substance of the lens, they undergo certain modi- 

 fications in refraction dependent upon the differences in the 

 various strata of the lens. These modifications have not been 

 accurately calculated ; but it is sufficient to state that they 

 contribute to the accuracy of the formation of the retinal 

 image and to the production of an image practically free from 

 chromatic dispersion. As the rays pass out of the crystalline 

 lens, they are again refracted by its posterior curvature, and 

 are brought to a focus at the area of distinct vision. 



The rays from all points of an object distinctly seen are 

 brought to a focus, if the accommodation of the lens be cor- 

 rect, upon a restricted surface in the macula lutea ; but the 

 rays from different points cross each other before they reach 

 the retina, and the image is consequently inverted. This is 

 a fact capable of actual demonstration, as we have shown in 

 treating of the formation of images in the eye. 



1 HELMHOLTZ, Optique physiologique, Paris, 1867, p. 89. 



