MECHANISM OF EEFEACTION IN THE EYE. 703 



lens, is the fovea centralis, which is the centre of distinct vision. . The an- 

 atomical elements of the fovea are capable of receiving visual impressions, 

 which are conveyed to the brain by the optic nerves. All impressions made 

 upon other portions of the retina are comparatively indistinct ; and the point 

 of entrance of the optic nerve is insensible to light. Inasmuch as the punc- 

 tum caecum is situated in either eye upon the nasal side of the retina, in nor- 

 mal vision, rays from the same object can not fall upon both blind points at 

 the same time. Thus, in binocular vision, the insensibility of the punctum 

 caecum does not interfere with sight ; and the movements of the globe pre- 

 vent any notable interference in vision, even with one eye. The sclerotic 

 coat is for the protection of its contents and for the insertion of muscles. 

 The iris has an action similar to that of the diaphragm in optical instru- 

 ments. The suspensory ligament of the lens, the ciliary body, and the cili- 

 ary muscle, are for the fixation of the lens and its accommodation for distinct 

 vision at different distances. The choroid is a dark membrane, for the ab- 

 sorption of light, preventing 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 -^ of an inch (10 mm.) behind the 

 retina. Without the crystalline lens, therefore, distinct, unaided vision gen- 

 erally is impossible, although the sensation of light is appreciated. In cases 

 of extraction of the lens for cataract (aphakia), 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 crystalline lens, by which they are 

 still farther refracted. Passing through the substance of the lens, they 

 undergo certain modifications in refraction, dependent upon the differences 

 in the various strata of the lens. These modifications have not been accu- 

 rately 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 point 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 correct, 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 inverted. 



Calculating the curvatures of the refracting surfaces in the eye and the 

 indices of refraction of its transparent media, it has been pretty clearly 

 shown, by mathematical formulae, that the eye viewed simply as an optical 

 instrument, and not practically, as the organ of vision presents a certain 

 degree of spherical and chromatic aberration ; but these calculations are not 

 very important in a purely physiological consideration of the sense of sight. 



In most calculations of the size of images, the positions of conjugate foci, 

 etc., in normal and abnormal eyes v a schematic eye reduced by Donders, after 



