680 SPECIAL SENSES 



either eye upon the nasal side of the retina, in normal vision, rays from 

 the same object can not fall on 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 prevent 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 instruments. 

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

 muscle are for the fixation of the lens and its accommodation for vision 

 at different distances. The pigment-cells are for the absorption of 

 light, preventing confusion of vision from reflection within the eye. 

 They also produce the visual purple. 



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 milli- 

 meters) behind the retina. Without the crystalline lens, therefore, dis- 

 tinct unaided vision usually is impossible, although the sensation of light 

 is appreciated. In cases of extraction of the lens for cataract (aphakia), 

 the place of the crystalline may be 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 further refracted. Passing through the substance of the 

 lens, they undergo certain modifications in refraction, dependent on 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 a retinal image practically 

 free from chromatic dispersion. As the rays pass out of the 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 is correct, on a restricted sur- 

 face in the macula lutea ; but the rays from different points cross 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 important in a purely physiological considera- 

 tion of the sense of sight. 



